4. The motor, in this case two long skeins of rubber; long, because we
wish to be able to give our motor many turns, from 700 to, say, 1,000
as a limit, so that the duration of flight may be considerable.
[Illustration: FIG. 129.-Sections of backbone for model aeroplane.]
The Backbone.—For the backbone or central rod take a piece of pitch
pine or satin walnut 52 inches long, 5/8 inch deep, and 1/2 inch broad,
and plane it down carefully until it has a T-shaped section, as shown
in Fig. 129, and the thickness is not anywhere more than 1/8 inch. It
is quite possible to reduce the thickness to even 1/16 inch and still
have a sufficient reserve of strength to withstand the pull of 28
strands of 1/16-inch rubber wound up 1,000 times; but such a course is
not advisable unless you are a skilful planer and have had some
experience in model-making.
If you find the construction of the T-shaped rod too difficult, two
courses are open—
(l) To get a carpenter to do the job for you, or
(2) To give the rod the triangular section shown in Fig. 129, each side
of the equilateral triangle being half an inch long.
[Illustration: FIG. 150—Side elevation of model aeroplane.]
The top of the T or the base of the triangle, as the case may be, is
used uppermost. This rod must be pierced in three places for the
vertical masts employed in the bracing of the rod, trussing the main
plane, and adjusting the elevator. These are spaced out in Fig. 130,
which shows a side elevation of the model. Their sectional dimensions
are 1/16 by 1/4 inch; their respective lengths are given in Fig. 130.
Round the front edges and sharpen the rear.
In Fig. 130 is shown the correct attitude or standing pose necessary to
make the model rise quickly and sweep boldly up into the air without
skimming the ground for some 10 to 20 yards as so many models do. E is
the elevator (7 by 3 inches); A the main plane (5-1/2 by 29 inches); W
the wheels; and RS the rear skid, terminating in a piece of hooked
steel wire. The vertical bracing of these masts is indicated. The best
material to use for the purpose is Japanese silk gut, which is very
light and strong. To brace, drill a small, neat hole in the mast and
rod where necessary, pass through, and tie. Do the same with each one.
To return to the central mast, which must also form the chassis. This
is double and opened out beneath as shown in Fig. 131, yz being a
piece similar to the sides, which completes, the triangle x y z and
gives the necessary rigidity. Attach this piece by first binding to its
extremities two strips of aluminium, or by preference very thin tinned
iron, Tl and T2. Bend to shape and bind to xy, xz as shown in Fig. 131.
[Illustration: FIG. 131.—Front elevation of chassis.]
[Illustration: FIG. l32.-Wheel for model aeroplane chassis.]
[Illustration: FIG. 133.—Plan of model aeroplane.]
The Wheels and Chassis.—WW are the two wheels on which the model runs.
They are made of hollow brass curtain rings, 1 inch in diameter, such
as can be bought at four a penny. For spokes, solder two strips of thin
tinned iron to the rings, using as little solder as possible. (Fig.
132.) To connect these wheels with the chassis, first bind to the lower
ends of xy, xz two strips of thin tinned iron, T3 and T4, after
drilling in them two holes of sufficient size to allow a piece of steel
wire of “bonnet pin” gauge to pass freely, but not loosely, through
them. Soften the wire by making it red hot and allowing it to cool
slowly, and solder one end of this wire (which must be quite straight
and 5-1/4 inches long) to the centre of the cross pieces or spokes of
one wheel. Pass the axle through the holes in the ends of xy, xz, and
solder on the other wheel. Your chassis is then finished.
The rear skid (RS in Fig. 130) is attached to the central rod by
gluing, and drilling a hole through both parts and inserting a wooden
peg; or the upright may be mortised in. On no account use nail, tack,
or screw. Attach the vertical masts and the horizontal ones about to be
described by gluing and binding lightly with thread, or by neatly glued
strips of the Hart’s fabric used for the planes.
Horizontal Spars, etc.—To consider now the horizontal section or part
plan of the model, from which, to avoid confusion, details of most
vertical parts are omitted. Referring to Fig. 133, it will be seen that
we have three horizontal masts or spars—HS1, 4 inches; HS2, 6 inches;
and HS3, slightly over 12 inches long. The last is well steamed,
slightly curved and left to dry while confined in such a manner as to
conform to the required shape. It should so remain at least twenty-four
hours before being fixed to the model. All the spars are attached by
glue and neat cross bindings. If the central rod be of triangular
instead of T section, the join can be made more neatly. The same
remarks apply to the two 9 and 10 inch struts at the propeller end of
the rod, which have to withstand the pull of the rubber motor on PPl.
These two pieces will have a maximum strength and minimum weight if of
the T section used for the rod. If the work is done carefully, 1/4 inch
each way will be sufficient.
Main Plane and Elevator.—The framework of each plane is simply four
strips of satin walnut or other suitable wood, 1/4 inch broad and 1/16
inch or even less in thickness for the main plane, and about 1/16 by
1/16 inch for the elevator. These strips are first glued together at
the corners and left to set. The fabric (Hart’s fabric or some similar
very light material) is then glued on fairly tight—that is, just
sufficiently so to get rid of all creases. The main plane is then fixed
flat on to the top of the central rod by gluing and cross binding at G
and H. (A better but rather more difficult plan is to fasten the
rectangular frame on first and then apply the fabric.) The same course
is followed in dealing with the elevator, which is fixed, however, not
to the rod, but to the 4-inch horizontal spar, HS1, just behind it, in
such a manner as to have a slight hinge movement at the back. This
operation presents no difficulty, and may be effected in a variety of
ways. To set the elevator, use is made of the short vertical mast, M1.
A small hole is pierced in the front side of the elevator frame at Z,
and through this a piece of thin, soft iron wire is pushed, bent round
the spar, and tied. The other end of the wire is taken forward and
wrapped three or four times round the mast M1, which should have
several notches in its front edge, to assist the setting of the
elevator at different angles. Pull the wire tight, so that the elevator
shall maintain a constant angle when once set. H H1 is a piece of 25 to
30 gauge wire bent as shown and fastened by binding. It passes round
the front of the rod, in which a little notch should be cut, so as to
be able to resist the pull of the twin rubber motors, the two skeins of
which are stretched between H H1 and the hooks formed on the propeller
spindles. If all these hooks are covered with cycle valve tubing the
rubber will last much longer. The rubber skeins pass through two little
light wire rings fastened to the underside ends of HS2. (Fig. 133.)
The front skid or protector, FS, is made out of a piece of thin, round,
jointless cane, some 9 inches in length, bent round as shown in Fig.
134, in which A B represents the front piece of the T-shaped rod and x
y z a the cane skid; the portion x y passing on the near side of the
vertical part of the T, and z a on the far side of the same. At E and F
thread is bound right round the rod. Should the nose of the machine
strike the ground, the loop of cane will be driven along the underside
of the rod and the shock be minimized. So adjust matters that the skid
slides fairly stiff. Note that the whole of the cane is on the under
side of the top bar of the T.
[Illustration: FIG. 134.—Front skid and attachment to backbone.]
Bearings.—We have still to deal with the propellers and their bearings.
The last, TN and TNl (Fig. 133), are simply two tiny pieces of tin
about half a gramme in weight, bent round the propeller spar HS3 at B
and B1. Take a strip of thin tin 1/4 inch wide and of sufficient length
to go completely round the spar (which is 1/4 by 1/8 inch) and overlap
slightly. Solder the ends together, using a minimum amount of solder.
Now bore two small holes through wood and tin from rear to front, being
careful to go through the centre. The hole must be just large enough to
allow the propeller axle to run freely, but not loosely, in it.
Primitive though such a bearing may seem, it answers admirably in
practice. The wood drills out or is soon worn more than the iron, and
the axle runs quite freely. The pull of the motor is thus directed
through the thin curved spar at a point where the resistance is
greatest—a very important matter in model aeroplane construction. To
strengthen this spar further against torsional forces, run gut ties
from B and Bl down to the bottom of the rear vertical skid post; and
from B to B1 also pass a piece of very thin piano wire, soldered to the
tin strips over a little wooden bridge, Q, like a violin bridge, on the
top of the central rod, to keep it quite taut.
[Illustration: FIG. 135—“Centrale” wooden propeller.]
Propellers.—To turn now to the propellers. Unless the reader has
already had fair experience in making model propellers, he should
purchase a couple, one right-handed and one left-handed, as they have
to revolve in opposite directions. It would be quite impossible to give
in the compass of this article such directions as would enable a novice
to make a really efficient propeller, and it must be efficient for even
a decent flight with a self-launching model. The diameter of the two
propellers should be about 11-1/2 to 11-3/4 inches, with a pitch angle
at the extremities of about 25 to 30 degrees as a limit. The “centrale”
type (Fig. 135) is to be preferred. Such propellers can be procured at
Messrs. A. W. Gamage, Ltd., Holborn, E.C.; Messrs. T. W. K. Clarke and
Co., Kingston-on-Thames; and elsewhere.
For the particular machine which we are considering, the total weight
of the two propellers, including axle and hook for holding the rubber,
should not exceed 3/4 oz. This means considerable labour in cutting and
sandpapering away part of the boss, which is always made much too large
in propellers of this size. It is wonderful what can be done by care
and patience. The writer has in more than one case reduced the weight
of a propeller by more than one-half by such means, and has yet left
sufficient strength.
The combined axle and hook should be made as follows:—Take a piece of
thin steel wire, sharpen one end, and bend it as shown at C (Fig. 136).
Pass the end B through a tight-fitting hole in the centre of the small
boss of the propeller, and drive C into the wood. Solder a tiny piece
of 1/8-inch brass tubing to the wire axle at A, close up to the rubber
hook side of the propeller, and file quite smooth. The only things now
left to do are to bend the wire into the form of a hook (as shown by
the dotted line), and to cover this hook, as already advised, with a
piece of valve tubing to prevent fraying the rubber skeins.
[Illustration: FIG. 136.—Axle and hook for propeller.]
Weight.—The weight of a model with a T-shaped central rod 1/16 inch
thick should be 4-1/2 oz. Probably it will be more than this—as a
maximum let us fix 6 oz.—although 4-1/2 oz. is quite possible, as the
writer has proved in actual practice. In any case the centre of gravity
of the machine without the rubber motor should be situated 1 inch
behind the front or entering edge of the main plane. When the rubber
motor (14 strands of 1/16-inch rubber for each propeller, total weight
2 oz.) is in position, the centre of gravity will be further forward,
in front of the main plane. The amount of rubber mentioned is for a
total weight of 6-1/2 oz. If the weight of the model alone be 6 oz.,
you will probably have to use 16 strands, which again adds to the
weight, and makes one travel in a vicious circle. Therefore I lay
emphasis on the advice, Keep down the weight.
The front edge of the elevator should be set about 3/8 inch higher than
the back, and the model be tried first as a glider, with the rubber and
propellers in position. If it glides satisfactorily, wind up the motor,
say 500 turns, and launch by hand. When a good flight has been
obtained, and the correct angle of the elevator has been determined,
place the model on a strip of linoleum, wind up, and release the
propellers. The model should rise in its own length and remain in the
air (if wound up 900 turns) at least three quarters of a minute. Choose
a calm day if possible. If a wind blows, let the model face the breeze.
Remember that the model flies high, and select a wide open space. Do
not push the model forward; just release the propellers, held one in
each hand near the boss by the fingers and thumb. As a lubricant for
the rubber use pure glycerine. It is advisable to employ a geared-up
mechanical winder, since to make 1,800 turns with the fingers is rather
fatiguing and very tedious.
Simple as this model may seem in design, one built by the writer on
exactly the lines given has met the most famous flying models of the
day in open competition and proved successful against them.
XXVI.
APPARATUS FOR SIMPLE SCIENTIFIC EXPERIMENTS.
Colour Discs for the Gramophone.—The gramophone, by virtue of its table
revolving at a controllable speed, comes in useful for a series of
optical experiments made with coloured discs bearing designs of
different kinds.
The material needed for these discs is cardboard, covered with white
paper on one side, or the Bristol board used by artists. The discs on
which the designs are drawn should be made as large as the gramophone
table will take conveniently, so as to be viewed by a number of people
at once. To encourage readers who do not possess a gramophone, it may
be pointed out that a gramophone, is merely a convenience, and not
indispensable for turning the discs, which may be revolved on a
sharpened pencil or any other spindle with pointed ends.
The Vanishing Spirals (Fig. 137).—This design, if spun slowly in a
clockwise direction, gives one the impression that the lines all move
in towards the centre. If the disc is turned in an anti-clockwise
direction, the lines seem to move towards the circumference and
disappear. To get the proper effect the gaze should be fixed and not
attempt to follow the lines round.
[Illustration: FIG. 137.]
[Illustration: FIG. 138.]
The Rolling Circles.—Figs. 138 and 139 are variations of the same idea.
In Fig. 138 two large circles are described cutting one another and
enclosing a smaller circle concentric with the disc. When spun at a
certain rate the larger circles will appear to run independently round
the small. The effect is heightened if the circles are given different
colours. If black only is used for the large circles, the eyes should
be kept half closed. In Fig. 139 two pairs of circles are described
about two centres, neither of which is the centre of the disc. The
pairs appear to roll independently.
[Illustration: FIG. 139.]
[Illustration: FIG. 140.]
The Wriggling Line (Fig. 140).—If this design is revolved at a low
speed and the eye is fixed on a point, the white (or coloured) line
will seem to undulate in a very extraordinary manner. The line is made
up of arcs of circles, and as the marking out is somewhat of a
geometrical problem, a diagram (Fig. 141) is added to show how it is
done. The dotted curves are those parts of the circles which do not
enter into the design.
Begin by marking out the big circle A for the disc. The circumference
of this is divided into six equal parts (chord equal to radius), and
through the points of division are drawn the six lines from the centre.
Describe circles aaa, each half the diameter of A. The circles bbb are
then drawn from centres on the lines RRR, and with the same radius as
aaa., The same centres are used for describing the circles a1 a1 a1 and
b1 b1 b1, parts of which form the inner boundary of the line. The
background should be blackened and the belt left white or be painted
some bright colour.
[Illustration: FIG. 141.]
Another optical illusion is afforded by Fig. 142. Two sets of circles
are described about different centres, and the crescent-shaped areas
between them coloured, the remainder of the disc being left white. The
disc is revolved about the centre of the white areas, and one gets the
impression that the coloured parts are portions of separate discs
separated by white discs.
[Illustration: FIG. 142.]
[Illustration: FIG. 143.]
The Magic Spokes (Fig. 143).—Place a design like this on the gramophone
and let it turn at high speed. The radial lines seem but a blur. Now
punch a hole one-eighth of an inch in diameter in a piece of blackened
card, and, standing well away from the gramophone, apply your eye to
the hole and move the card quickly to and fro. The extreme briefness of
the glimpses obtained of the moving lines seems to rob them of motion,
or even make them appear to be moving in the direction contrary to the
actual. Instead of a single hole, one may use a number of holes punched
at equal intervals round a circle, and revolve the card on the centre.
If a certain speed be maintained, the spokes will appear motionless.
The substitution of a long narrow slit for a circular hole gives other
effects.
[Illustration: FIG. 144.]
A Colour Top.—Cut a 4-inch disc out of white cardboard and blacken
one-half with Indian ink. On the other half draw four series of
concentric black lines, as shown in Fig. 144. If the disc is mounted on
a knitting needle and spun in a horizontal plane, the black lines will
appear of different colours. A clockwise rotation makes the outermost
lines appear a greenish blue, those nearest the centre a dark red, and
the intermediate groups yellow and green. A reversal of the motion
reverses the order of the colours, the red lines now being farthest
from the centre. The experiment is generally most successful by
artificial light, which contains a larger proportion of red and yellow
rays than does sunlight. The speed at which the top revolves affects
the result considerably. It should be kept moderate, any excess tending
to neutralize the colours.
[Illustration: FIG. 145.]
The Magic Windmill.—Mark a circle 2-1/2 inches in diameter on a piece
of notepaper, resting the centre leg [of the compass] so lightly that
it dents without piercing the paper. With the same centre describe a
3/4-inch circle. Join the circles by eight equally spaced radial lines,
and an eighth of an inch away draw dotted parallel lines, all on the
same side of their fellow lines in order of rotation. Cut out along the
large circle, and then with a. sharp knife follow the lines shown
double in Fig. 145. This gives eight little vanes, each of which must
be bent upwards to approximately the same angle round a flat ruler held
with an edge on the dotted line. Next make a dent with a lead pencil at
the exact centre on the vane side, and revolve the pencil until the
dent is well polished.
[Illustration: FIG. 146.]
Hold a pin, point upwards, in the right hand, and with the left centre
the mill, vanes pointing downwards, on the pin (Fig. 146). The mill
will immediately commence to revolve at a steady pace, and will
continue to do so indefinitely; though, if the head of the pin be stuck
in, say, a piece of bread, no motion will occur. The secret is that the
heat of the hand causes a very slight upward current of warmed air,
which is sufficient to make the very delicately poised windmill
revolve.
A Pneumatic Puzzle.—For the very simple apparatus illustrated by Fig.
147 one needs only half a cotton reel, three pins, and a piece of glass
or metal tubing which fits the hole in the reel. Adjust a halfpenny
centrally over the hole and stick the pins into the reel at three
equidistant points, so that they do not quite touch the coin, and with
their ends sloping slightly outwards to allow the halfpenny to fall
away.
[Illustration: FIG. 147.—Apparatus for illustrating an apparent
scientific paradox.]
Press the coin against the reel and blow hard through the tube. One
would expect the coin to fall; but, on the contrary, the harder you
blow the tighter will it stick, even if the reel be pointed downwards.
Only when you stop blowing will it fall to the floor.
This is a very interesting experiment, and will mystify onlookers who
do not understand the reason for the apparent paradox, which is this.
The air blown through the reel strikes a very limited part of the
nearer side of the halfpenny. In order to escape, it has to make a
right-angle turn and pass between coin and reel, and, while travelling
in this direction, loses most of its repulsive force. The result is
that the total pressure on the underside of the coin, plus the effect
of gravity, is exactly balanced by the atmospheric pressure on the
outside, and the coin remains at that distance from the reel which
gives equilibrium of forces. When one stops blowing, the air pressure
on both sides is the same, and gravity makes the coin fall away.
The function of the pins is merely to keep the halfpenny centred on the
hole. If steam is used instead of human breath, a considerable weight
may be hung from the disc without dislodging it.
The Magic Swingers.—The easily made toy illustrated next is much more
interesting than would appear from the mere picture, as it demonstrates
a very striking physical phenomenon, the transference of energy. If two
pendulums are hung close together from a flexible support and swung,
their movements influence one another in a somewhat remarkable way—the
swing of the one increasing as that of the other dies down, until a
certain point is reached, after which the process is reversed, and the
“dying” or “dead” pendulum commences to come to life again at the
expense of the other. This alternation is repeated over and over again,
until all the energy of both pendulums is exhausted.
[Illustration: FIG. 148.-Magic pendulums.]
To make the experiment more attractive, we substitute for the simplest
possible pendulums—weights at the end of strings—small swings, each
containing a figure sitting or standing on a seat, to the underside of
which is attached a quarter of a pound of lead. To prevent the swings
twisting, they are best made of strong wire bent as shown in Fig. 148,
care being taken that the sides are of equal length, so that both hooks
may press equally on the strings. Eighteen inches is a good length. The
longer the swing, and the heavier the weight, the longer will the
experiment last.
The swings are hung, six inches apart, from a stout string stretched
tightly between two well-weighted chairs or between two fixed points.
The string should be at least 4 feet long.
With two equally long and equally weighted pendulums, the three
following experiments may be carried out:—
1. Let one, A, start from rest. The other, B will gradually die, and A
swing to and fro more and more violently, till B at last comes to a
dead stop. Then A will die and B in turn get up speed. The energy
originally imparted to B is thus transferred through the string from
one pendulum to the other an indefinite number of times, with a slight
loss at every alternation, until it is finally exhausted by friction.
2. Swing them in opposite directions, but start A from a higher point
than B. They will each alternately lose and gain motion, but will never
come to rest, and will continue to swing in opposite directions—that
is, while A swings north or east B will be swinging south or west, and
vice versa.
3. Start them both in the same direction, but one from a higher point
than the other. There will be the same transference of energy as in
(2), but neither will come to rest between alternations, and they will
always swing in the same direction.
Unequal Lengths.—If for one of the original pendulums we substitute one
a couple of inches longer than the other, but of the same weight, the
same set of three experiments will provide six variations among them,
as in each case either the longer or the shorter may be started first
or given the longer initial swing, as the case may be. The results are
interesting throughout, and should be noted.
Three or more Pendulums.—If the number of pendulums be increased to
three or more, the length of all being the same, a fresh field for
observation is opened. With an increase of number a decrease in the
individual weighting is advisable, to prevent an undue sagging of the
string.
In conclusion, we may remark that a strong chain stretched between two
trees and a suitable supply of rope will enable the reader and his
friends to carry out all the experiments on a life-size scale.
A Smoke-ring Apparatus.—Get a large tin of the self-opening kind and
cut a hole 2 inches across in the bottom. Then make a neat circular
hole 1-1/4 inches in diameter in the centre of a paper disc somewhat
smaller than the bottom of the tin, to which it is pasted firmly on the
outside. The other end—from which the lid is removed—must be covered
with a piece of sheet rubber stretched fairly tight and secured to the
tin by string passed over it behind the rim. An old cycle or motor car
air tube, according to the size of the tin, will furnish the rubber
needed; but new material, will cost only a few pence (Fig. 149).
[Illustration: FIG. 149.—Smoke-ring apparatus.]
A dense smoke is produced by putting in the tin two small rolls of
blotting paper, one soaked in hydrochloric acid, the other in strong
ammonia. The rolls should not touch. To reduce corrosion of the tin by
the acid, the inside should be lined with thin card.
[Illustration: FIG. 150.—Smoke-making apparatus.]
A ring of smoke is projected from the hole in the card if the rubber
diaphragm is pushed inwards. A slow, steady push makes a fat, lazy ring
come out; a smart tap a thinner one, moving much faster. Absolutely
still air is needed for the best effects, as draughts make the rings
lose shape very quickly and move erratically. Given good conditions, a
lot of fun can be got out of the rings by shooting one through another
which has expanded somewhat, or by destroying one by striking it with
another, or by extinguishing a candle set up at a distance, and so on.
The experimenter should notice how a vortex ring rotates in itself
while moving forward, like a rubber ring being rolled along a stick.
A continuous supply of smoke can be provided by the apparatus shown in
Fig. 150. The bulb of a scent spray is needed to force ammonia gas
through a box, made air-tight by a rubber band round the lid, in which
is a pad soaked with hydrochloric acid. The smoke formed in this box is
expelled through a pipe into the ring-making box.
Caution.—When dealing with hydrochloric acid, take great care not to
get it on your skin or clothes, as it is a very strong corrosive.
XXVII.
A RAIN-GAUGE.
The systematic measurement of rainfall is one of those pursuits which
prove more interesting in the doing than in the prospect. It enables us
to compare one season or one year with another; tells us what the
weather has been while we slept; affords a little mild excitement when
thunderstorms are about; and compensates to a limited extent for the
disadvantages of a wet day.
The general practice is to examine the gauge daily (say at 10 a.m.); to
measure the water, if any, collected during the previous twenty-four
hours; and to enter the record at once. Gauges are made which record
automatically the rainfall on a chart or dial, but these are
necessarily much more expensive than those which merely catch the water
for measurement.
This last class, to which our attention will be confined chiefly, all
include two principal parts—a metal receiver and a graduated glass
measure, of much smaller diameter than the receiver, so that the
divisions representing hundredths of an inch may be far enough apart to
be distinguishable. It is evident that the smaller the area of the
measure is, relatively to that of the receiver, the more widely spaced
will the graduation marks of the measure be, and the more exact the
readings obtained.
[Illustration: FIG. 151.—Standard rain-gauge.]
The gauge most commonly used is that shown in Fig. 151. It consists of
an upper cylindrical part, usually 5 or 8 inches in diameter, at the
inside of the rim, with its bottom closed by a funnel. The lower
cylindrical part holds a glass catcher into which the funnel delivers
the water for storage until the time when it will be measured in a
graduated glass. The upper part makes a good fit with the lower, in
order to reduce evaporation to a minimum.
Such a gauge can be bought for half a guinea or so, but one which, if
carefully made, will prove approximately accurate, can be constructed
at very small expense. One needs, in the first place, a cylindrical
tin, or, better still, a piece of brass tubing, about 5 inches high and
not less than 3 inches in diameter. (Experiments have proved that the
larger the area of the receiver the more accurate are the results.) The
second requisite is a piece of stout glass tubing having an internal
diameter not more than one-quarter that of the receiver This is to
serve as measuring glass.
[Illustration: FIG. 152.—Section of homemade rain-gauge.]
The success of the gauge depends entirely upon ascertaining accurately
how much of the tube will be filled by a column of water 1 inch deep
and having the same area as the receiver. This is easily determined as
follows:—If a tin is to be used as receiver, make the bottom and side
joints watertight with solder; if a tube, square off one end and solder
a flat metal to it temporarily. The receptacle is placed on a perfectly
level base, and water is poured in until it reaches exactly to a mark
made 4 inches from the end of a fine wire held perpendicularly. Now
cork one end of the tube and pour in the water, being careful not to
spill any, emptying and filling again if necessary. This will give you
the number of tube inches filled by the 4 inches in the receiver.
Divide the result by 4, and you will have the depth unit in the measure
representing 1 inch of rainfall. The measuring should be done several
times over, and the average result taken as the standard. If the
readings all agree, so much the better.
Preparing the Scale.—The next thing is to graduate a scale, which will
most conveniently be established in indelible pencil on a carefully
smoothed strip of white wood 1 inch wide. First make a zero mark
squarely across the strip near the bottom, and at the unit distance
above it a similar mark, over which “One Inch” should be written
plainly. The distance between the marks is next divided by 1/2-inch
lines into tenths, and these tenths by 1/4-inch lines into hundredths,
which, if the diameter of the receiver is four times that of the tube,
will be about 3/16 inch apart. For reading, the scale is held against
the tube, with the zero mark level with the top of the cork plugging
the bottom. It will, save time and trouble if both tube and scale are
attached permanently to a board, which will also serve to protect the
tube against damage.
Making the Receiver.—A tin funnel, fitting the inside of the receiver
closely, should be obtained, or, if the exact article is not available,
a longer one should be cut down to fit. Make a central hole in the
bottom of the receiver large enough to allow the funnel to pass through
up to the swell, and solder the rim of the funnel to the inside of the
receiver, using as little heat as possible.
If you select a tin of the self-opening kind, you must now cut away the
top with a file or hack-saw, being very careful not to bend the metal,
as distortion, by altering the area of the upper end of the tin, will
render the gauge inaccurate.
The receiver should be supported by another tin of somewhat smaller
diameter, and deep enough to contain a bottle which will hold 3 or 4
inches of rainfall. In order to prevent water entering this
compartment, tie a strip of rubber (cut out of an old cycle air tube)
or other material round the receiver, and projecting half an inch
beyond the bottom (Fig. 152).
All tinned iron surfaces should be given a couple of thin coats or
paint.
The standard distance between the rain gauge and the ground is one
foot. The amount caught decreases with increase of elevation, owing to
the greater effect of the wind. The top of the gauge must be perfectly
level, so that it may offer the same catchment area to rain from
whatever direction it may come.
[Illustration: FIG. 153.—Self-measuring gauge.]
Another Arrangement.—To simplify measurement, the receiver and tube may
be arranged as shown in Fig. 153. In this case the water is delivered
directly into the measure, and the rainfall may be read at a glance. On
the top of the support is a small platform for the receiver, its centre
directly over the tube. The graduations, first made on a rod as already
described, may be transferred, by means of a fine camel’s hair brush
and white paint, to the tube itself. To draw off the water after taking
a reading, a hole should be burnt with a hot wire through the bottom
cork. This hole is plugged with a piece of slightly tapered brass rod,
pushed in till its top is flush with the upper surface of the cork.
If the tube has small capacity, provision should be made for catching
the overflow by inserting through the cork a small tube reaching to a
convenient height-say the 1-inch mark. The bottom of the tube projects
into a closed storage vessel. Note that the tube must be in position
before the graduation is determined, otherwise the readings will
exaggerate the rainfall.
[Illustration: FIG. 154.—Gauge in case.]
Protection against the Weather.—A rain-gauge of this kind requires
protection against frost, as the freezing of the water would burst the
tube. It will be sufficient to hinge to the front of the support a
piece of wood half an inch thicker than the diameter of the tube,
grooved out so as to fit the tube when shut round it (Fig 154).
XXVIII.
WIND VANES WITH DIALS.
It is difficult to tell from a distance in which direction the arrow of
a wind vane points when the arrow lies obliquely to the spectator, or
points directly towards or away from him. In the case of a vane set up
in some position where it will be plainly visible from the house, this
difficulty is overcome by making the wind vane operate an arrow moving
round a vertical dial set square to the point of observation. Figs. 155
to 157 are sketches and diagrams of an apparatus which does the work
very satisfactorily. The vane is attached to the upper end of a long
rod, revolving freely in brackets attached to the side of a pole. The
bottom end of the rod is pointed to engage with a nick in a bearer, in
which it moves with but little friction. Near the end is fixed a
horizontal bevel-wheel, engaging with a vertical bevel of equal size
and number of teeth attached to a short rod running through a hole in
the post to an arrow on the other side. Between arrow and post is room
for a dial on which the points of the compass are marked.
The construction of the apparatus is so simple as to call for little
comment. The tail of the vane is made of two pieces of zinc, tapering
from 8 inches wide at the rear to 4 inches at the rod, to which they
are clipped by 4 screws and nuts. A stay soldered between them near the
stern keeps the broader ends a couple of inches apart, giving to the
vane a wedge shape which is more sensitive to the wind than a single
flat plate. The pointer also is cut out of sheet metal, and is attached
to the tail by means of the screws already mentioned. It must, of
course, be arranged to lie in a line bisecting the angle formed by the
two parts of the tail.
[Illustration: FIG. 165—Wind vane with dial.]
The rod should preferably be of brass, which does not corrode like
iron. If the uppermost 18 inches or so are of 1/4-inch diameter, and
assigned a bracket some distance below the one projecting from the top
of the pole, the remainder of the rod need not exceed 1/8 to 5/32 inch
in diameter, as the twisting strain on it is small. Or the rod may be
built up of wooden rods, well painted, alternating with brass at the
points where the brackets are.
[Illustration: FIG. 156.—Elevation and plan of vane.]
The Bevel Gearing.—Two brass bevel wheels, about 1 inch in diameter,
and purchasable for a couple of shillings or less, should be obtained
to transmit the vane movements to the dial arrow. Grooved pulleys, and
a belt would do the work, but not so positively, and any slipping
would, of course, render the dial readings incorrect. The arrow spindle
(of brass) turns in a brass tube, driven tightly into a hole of
suitable size bored through the centre of the post (Fig. 157). It will
be well to fix a little metal screen over the bevel gear to protect it
from the weather.
[Illustration: FIG. 157.—Details of bevel gear and arrow.]
The Dial—This is made of tinned iron sheet or of 1/4-inch wood nailed
to 1/2-inch battens. It is held up to the post by 3-inch screws passing
through front and battens. At the points of contact, the pole is
slightly flattened to give a good bearing; and, to prevent the dial
being twisted off by the wind, strip iron or stout galvanized wire
stays run from one end of a batten to the other behind the post, to
which they are secured.
The post should be well painted, the top protected by a zinc disc laid
under the top bracket, and the bottom, up to a point 6 inches above the
ground level, protected by charring or by a coat of boiled tar, before
the dial and the brackets for the vane rod to turn in are fastened on.
A white dial and black arrow and letters will be most satisfactory
against a dark background; and vice versa for a light background. The
letters are of relatively little importance, as the position of the
arrow will be sufficient indication.
It gives little trouble to affix to the top of the pole 4 arms, each
carrying the initial of one of the cardinal points of the compass. The
position of these relatively to the direction in which the dial will
face must be carefully thought out before setting the position in the
ground. In any case the help of a compass will be needed to decide
which is the north.
Having set in the post and rammed the earth tightly round it, loosen
the bracket supporting the vane rod so that the vane bevel clears the
dial bevel. Turn the vane to true north, set the dial arrow also to
north, and raise the bevel so that it meshes, and make the bracket
tight.
Note.—In the vicinity of London true north is 15 degrees east of the
magnetic north.
The pole must be long enough to raise the vane clear of any objects
which might act as screens, and its length will therefore depend on its
position. As for the height of the dial above the ground, this must be
left to individual preference or to circumstances. If conditions allow,
it should be near enough to the ground to be examined easily with a
lamp at night, as one of the chief advantages of the system is that the
reading is independent of the visibility of the vane.
A Dial Indoors.—If some prominent part of the house, such as a chimney
stack, be used to support the pole—which in such a case can be quite
short—it is an easy matter to connect the vane with a dial indoors,
provided that the rod can be run down an outside wall.
An Electrically Operated Dial.—Thanks to the electric current, it is
possible to cause a wind vane, wherever it may be set, to work a dial
situated anywhere indoors. A suggested method of effecting this is
illustrated in Figs. 158 to 161, which are sufficiently explicit to
enable the reader to fill in details for himself.
[Illustration: FIG. 158.—Plan and elevation of electric contact on vane
post.]
In-this case the vane is attached (Fig. 158) to a brass tube, closed at
the upper end, and supported by a long spike stuck into the top of the
pole. A little platform carries a brass ring, divided into as many
insulated segments as the points which the vane is to be able to
register. Thus, there will be eight segments if the half-points as well
as the cardinal points are to be shown on the dial. The centre of each
of these segments lies on a line running through the centre of the
spike to the compass point to which the segment belongs. The tube moves
with it a rotating contact piece, which rubs against the tops of the
segments.
Below it is a “brush” of strip brass pressing against the tube. This
brush is connected with a wire running to one terminal of a battery
near the dial.
[Illustration: FIG. 159.—Magnetic recording dial.]
The Dial.—This may be either vertical or horizontal, provided that the
arrow is well balanced. The arrow, which should be of some light
non-magnetic material, such as cardboard or wood, carries on its lower
side, near the point, a piece of soft iron. Under the path of this
piece is a ring of equally spaced magnets, their number equaling that
of, the segments on the vane. Between arrow and magnets is the dial on
which the points are marked (Fig. 159).
Each segment is connected by a separate wire with the corresponding
dial magnet, and each of these, through a common wire and switch, with
the other terminal of the battery (Fig. 161).
In order to ascertain the quarter of the wind, the switch is closed.
The magnet which is energized will attract the needle to it, showing in
what direction the vane is pointing. To prevent misreading, the dial
may be covered by a flap the raising of which closes the battery
circuit. A spring should be arranged to close the flap when the hand is
removed, to prevent waste of current.
[Illustration: FIG. 160.—Another type of electric dial with compass
needle for pointer.]
The exactitude of the indication given by the arrow depends on the
number of vane segments used. If these are only four, a N. read- ing
will be given by any position of the vane between N.E. and N.W.; if
eight, N. will mean anything between N.N.E. and N.N.W. Telephone
cables, containing any desired number of insulated wires, each covered
by a braiding of a distinctive colour, can be obtained at a cost only
slightly exceeding that of an equal total amount of single insulated
wire. The cable form is to be preferred, on account of its greater
convenience in fixing.
The amount of battery power required depends on the length of the
circuit and the delicacy of the dial. If an ordinary compass needle be
used, as indicated in Fig. 160, very little current is needed. In this
case the magnets, which can be made of a couple of dozen turns of fine
insulated wire round a 1/8-in soft iron bar, should be arranged
spokewise round the compass case, and care must be taken that all the
cores are wound in the same direction, so as to have the same polarity.
Otherwise some will attract the N. end of the needle and others repel
it. The direction of the current flow through the circuit will decide
the polarity of the magnets, so that, if one end of the needle be
furnished with a little paper arrow-head, the “correspondence” between
vane and dial is easily established. An advantage attaching to the use
of a compass needle is that the magnet repels the wrong end of the
needle.
[Illustration: FIG. 161.—General arrangement of electric wind
recorder.]
The brush and segments must be protected from he weather by a cover,
either attached to the segment platform or to the tube on which the
vane is mounted.
The spaces between the segments must be filled in flush with some
non-conducting material, such as fibre, vulcanite, or sealing-wax; and
be very slightly wider than the end of the contact arm, so that two
segments may not be in circuit simultaneously. In certain positions of
the vane no contact will be made, but, as the vane is motionless only
when there is no wind or none to speak of, this is a small matter.
XXIX.
A STRENGTH-TESTING MACHINE.
The penny-in-the-slot strength-testing machine is popular among men and
boys, presumably because many of them like to show other people what
their muscles are capable of, and the opportunity of proving it on a
graduated dial is therefore tempting, especially if there be a
possibility of recovering the penny by an unusually good performance.
For the expenditure of quite a small number of pence, one may construct
a machine which will show fairly accurately what is the value of one’s
grip and the twisting, power of the arms; and, even if inaccurate, will
serve for competitive purposes. The apparatus is very simple in
principle, consisting of but five pieces of wood, an ordinary spring
balance registering up to 40 lbs., and a couple of handles. The total
cost is but a couple of shillings at the outside.
Fig. 162 is a plan of the machine as used for grip measuring. The base
is a piece of deal 1 inch thick, 2 feet long, and 5-1/2 inches wide.
The lever, L, is pivoted at P, attached to a spring balance at Q, and
subjected to the pull of the hand at a point, R.
The pressure exerted at R is to that registered at Q as the distance PQ
is to the distance PR. As the spring balance will not record beyond 40
lbs., the ratio of PQ to PR may conveniently be made 5 to 1, as this
will allow for the performances of quite a strong man; but even if the
ratio be lowered to 4 to 1, few readers will stretch the balance to its
limit.
The balance should preferably be of the type shown in Fig. 162, having
an indicator projecting at right angles to the scale through a slot, as
this can be very easily fitted with a sliding index, I, in the form of
a 1/4-inch strip of tin bent over at the ends to embrace the edges of
the balance.