clock-making, and the utmost care and skill are required to execute a
train which shall not only run with as little friction as possible,
but the friction must be equal; for if there is no variation in the
train force, the escapement and pendulum will always be actuated by the
same amount of power, and the performance of the clock can be relied
upon. Clock text-books do not fully impress this subject. We find a
great deal upon this or that escapement, and the different pendulums.
Dennison has a couple of pages full of abstruse calculations upon a
method of shifting an extra weight upon a rod, so that the going of a
clock can be varied one second per day; but if his wheels and pinions
are not perfect, a large tooth here and there will vary the clock more
than that.
Reid overawes us with his knowledge of the proper curves of the teeth
of wheels; but it must have been only theory, for his practice was to
saw his teeth, and his cycloids, epicycloids, and hypocycloids were
left to the mercy of the “topping file” in the hands of his “wheel
teeth finishers,” instead of shaping up the teeth in the engine, as is
done now. We have generally cut the wheels of fine clocks over several
times with different cutters before taking them from the engine; the
last cutter having but one tooth, which can be made perfect as to cut
and shape, and, running with great speed, will leave the teeth the
proper shape, very smooth, and as true as the dial of the engine.
Escape wheels, especially, require great care in cutting, as the
teeth for dead-beat escapements are somewhat long and thin; the least
inaccuracy is certain to cause trouble. It is absolutely necessary that
the dial plate of the cutting engine should be perfectly true, with
clean, round holes, and a perfect fitting index point, with a cutter
arbor without end play or lateral motion--these are the essentials of a
good cutting engine, without which a good clock cannot be made.
We have generally made a practice, upon the completion of the train for
a fine clock, to put in the place of the escape-wheel a very light,
well-balanced fly, to prevent “backlash,” and a very fine soft cord
on the barrel; then hang on a very light weight; so slight that--all
of the wheels being balanced, and no oil upon the pivots--the fly
will move so slowly that its revolutions may be counted. By taking
care that the weight be not too much in excess of the resistance, the
least inaccuracy in the wheels and pinions may be discovered by the
difference in the velocity of the fly, or by its suddenly stopping,
which will be occasioned by any inequality in the train teeth, which
would not have been discovered by the closest scrutiny. It was by means
of this test that we discovered an inaccuracy in a pinion, caused by
hardening, which could not have been discovered by a less delicate test.
The wheels in the train should be as light as possible, for as the
whole train is stopped every time a tooth drops on the pallets, it is
plain that the driving weight must overcome the inertia as well as the
friction of the train at every beat. To this end it has been customary
to “arm out” the wheels, leaving a very light rim supported by light
arms, the wheels being generally of cast brass, turned up, and cut,
then lightened. We followed this plan for some time, but abandoned it,
as we found great difficulty in making a perfectly round wheel. The
arms serve as posts to support the rim in cutting or turning, but the
space between is very apt to spring down. We prefer making the wheels
of fine hard-rolled sheet brass; it is superior to cast brass, much
finer, harder, and more durable, and is freer from flaws. After the
wheels are cut, they are turned out on each side, leaving a thin web in
the centre; they can be made lighter, finished easier, and are round.
As to the shape of the teeth in clock-wheels, the subject has been so
ably treated by Reid, Dennison, and Prof. Willis (who has invented an
instrument to assist in laying out the curves for the teeth of wheels),
that we shall not attempt it in this paper; besides, there is so little
of the entire theory that can be applied to a clock-wheel of two and
a half inches in diameter, with 120 to 140 teeth, farther than to
leave the wheel and pinion of the proper diameter, that we consider it
unnecessary; for if makers of regulators and other fine clocks will
use pinions of 16 or 20 teeth, the friction or driving is all after the
line of centres, and the whole subject of cycloids, epicycloids, and
hypocycloids is reduced to a very small point, and might be said to
“vanish into thin air.”
Having given only a few practical hints, and not yet crossed the
threshold of the subject, we propose to continue from month to
month--if the readers of the JOURNAL do not weary--the discussion of
the various parts that go to make the sum total of a fine clock, with
notices of the various clocks made in this country.
* * * * *
It certainly comes within the province, and is the duty, of a journal
devoted to Horology, to make a note of any and all the new improvements
that pertain to the science. We give, then, some few, the merits of
which have struck us as being a very important matter of consideration.
The best clock time-keeper is not absolutely perfect, so its rate must
be kept; but the watchmaker ordinarily has no means of correcting the
error of his regulator, until the accumulation renders it a serious
inconvenience. Did he possess a Transit instrument, properly set and
adjusted for meridian, together with the required books and knowledge
of observing, he could from day to day correct his clock and keep
accurate time; but these are all expensive, as well as involving time
and labor. Suited to the wants of the artisan is a little instrument
called the Dipleidescope; simple in its construction, and not liable
to get out of position or order, it forms the best substitute for the
transit we have seen. It is founded on the theory that the double
reflection from the two surfaces of planes at an angle of 60° will
coincide when the object reflected is in a true line with half the base
of the whole triangle. Having a prism cut in an equilateral triangle,
one angle is set directly down toward the centre of the earth, the base
being brought parallel with the line of the horizon. Now, if the axis
of the prism is in a line with the meridian, a reflection of the sun
will appear, at the instant of crossing the meridian, on itself--that
is, there would be but one image. If the instrument is well made,
there can be no doubt of its accuracy and value to those who, wishing
to verify their time, are not situated so as to use a transit.
Another improvement is a Bench-Key for watchmaker’s use. No one who has
had any experience at the bench but will appreciate an article that
facilitates the setting of time-pieces for his customers. In winding,
it is equally valuable. It is not dependent for its strength of torsion
on the spring-chuck principle, the power being applied close to the
square by means of a pin that passes through the key.
Hall’s Patent Cutting Nippers are a positive desideratum; a large
wire can be cut off without the least jar to the hand, the leverage
is so great. The smallest sizes are suitable to the ordinary run
of watch-work, and can be used in clock-work better than any
cutting-plyers extant. Strong and durable, they possess one quality
that all watchmakers will appreciate--if a cutting-jaw is broken it can
be replaced by another.
Greenwich Observatory.
About two hundred years ago, England began to take a lead in the
mercantile commerce of the world; her ships were daily passing across
the Atlantic, and India also was beginning to attract her attention.
It was therefore of the utmost importance that navigators should be
enabled to find their longitude when at sea, independently of watches
or clocks; and a reward was offered to any one who should discover a
method by which this result might be obtained.
The plan proposed was, that the angular distance of the moon from
certain stars should be calculated beforehand, and published, so that,
for example, it might be stated that at ten minutes and five seconds
past nine on such a day, the moon should be distant from Mars 40
degrees. If from a ship in the middle of the Atlantic, Mars and the
moon were found to be 40 degrees apart, then it would be known that the
time in England was ten minutes and five seconds past nine.
Here, then, was one item ascertained, and the method was a good one;
but in consequence of the want of accuracy as regarded the moon’s
motions, and the exact positions of the stars, it could not be
practically carried out.
Under these circumstances, Charles II. decided that a national
observatory should be built, and an astronomer appointed; and a site
was at once selected for the building. Wren, the architect, selected
Greenwich Park as the most suitable locality, because from thence
vessels passing up and down the Thames might see the time-signals,
and also because there was a commanding view north and south from
the hill selected for the site. The observatory was completed in
1676, and Flamsteed, the chief astronomer, immediately commenced his
observations, but with very imperfect instruments of his own. During
thirty years, Flamsteed labored indefatigably, and formed a valuable
catalogue of stars, and made a vast collection of lunar observations.
He was succeeded by Halley, who carried on similar observations; and
from that time to the present, Greenwich Observatory has been our
head-quarters for astronomical observations.
The work carried on at Greenwich is entirely practical, and consists
in forming a catalogue of stars and planets, and so watching them
that every change in their movements is at once discovered. Now that
this work has been performed for several years, the movements of the
principal celestial bodies have been so accurately determined, that the
_Nautical Almanac_--the official guide on these subjects--is published
four years in advance, and thus we find that on a particular night in
1868, the moon will be at a certain angular distance from a star, and
the second satellite of Jupiter will disappear at a particular instant.
On the exterior wall of the observatory there is a large electric
clock, which, being placed in “contact” with the various other clocks
in the observatory, indicates exact Greenwich time. The face of this
clock shows twenty-four hours, so that it requires that a novice should
look at it twice before comparing his watch. On the left of this clock
are metal bars let into the wall, each of which represents the length
of a standard measure, such as a yard, foot, etc. And let us here say
a few words about these standards. To the uninitiated a yard is simply
three feet, and a foot is twelve inches--an inch being, we are told in
our “Tables,” the length of three barleycorns. Now, as the length of
a barleycorn varies considerably, it requires something more definite
than this to determine our national measures. Thus, the question, what
_is_ a foot? is more difficult to answer than at first sight appears.
Many years ago the French perceived the difficulty appertaining to the
national standard, and they, therefore, decided that a metre should be
the ten-millionth part of one-fourth of the earth’s circumference--that
is, ten-millionth of the distance from the Equator to the Pole. But
here another difficulty was encountered, because different calculators
found this arc of different lengths. By _law_, however, it was decided
that one measurement only was correct, and so the metre was fixed at
3.0794 Paris feet; though since then, more accurate observations and
improved instruments have shown these measured acres to have been
very incorrectly ascertained, and thus the French method failed when
practically tried.
The length of a seconds pendulum oscillating in a certain latitude has
been our method of obtaining a standard; but this also has its weak
points, so that to obtain a constant standard it is necessary to have
some pattern which is unchangeable, and thus a metal has been chosen
that expands or contracts but little either with heat or cold; and
this, at a certain temperature, is _the_ standard measure, and such a
standard may be seen on the exterior wall of Greenwich Observatory.
On entering the doorway--which is guarded by a Greenwich pensioner,
who will possibly first peep at the visitor, in order to see who
the individual may be who is desirous to tread within the sacred
precincts--one finds a court-yard, on the left of which are the
transit-room, the computing-room, and the chronometer-room. The
transit room takes its name from the instrument therein, which is
a large “transit.” This consists of a large telescope, the outside
of which is not unlike a heavy cannon, as it is of solid iron. The
instrument is supported by trunnions, which allow the telescope to be
elevated or depressed to point south or north, and, in fact, to make
a complete revolution, but never to diverge from the north or south
line. The magnifying power of this instrument is not very great, so
that it admits plenty of light, for it is intended, not as a searcher
for or for gazing at celestial objects, but for the purpose of noting
the exact time at which stars and planets pass south or north of
Greenwich. Upon looking through this telescope, the observer’s eye
is first attracted by a vertical row of what seem to be iron bars,
placed at equal distances from each other. These, however, prove to be
only spiders’ webs, and are used for the purpose of taking the time
of passage of a star over each wire, and thus to ascertain the exact
instant of its being in the centre of the telescope. During even the
finest and calmest nights, there is occasionally found a tremulousness
in the instrument, which, as it is rigidly fixed to the walls of the
building, must be due to a slight vibration in the ground itself. Thus,
many a feeble earthquake unfelt by the outsider may be perceived by the
astronomer by the aid of his delicate instruments.
The various stars seem to be travelling at an immense rate when
seen in the field of the transit telescope, and it is really nervous
work noting the exact time when each wire is passed. The experienced
observer, however, not only will give the minute and second, but also
the decimal of a second when the star was on the wire. The result is
obtained by counting the beats of a clock the face of which is opposite
the observer. Thus, if at three the star seems as much short of the
wire as at four it had passed it, then 3.5 might be the instant of
“transit.”
At noon each day the sun’s passage is observed by nearly the whole
staff of observers. One individual looks through the telescope, and
gives the time for each wire, while others examine a variety of
micrometers in order to ascertain the fractional parts of seconds,
etc.,--these micrometers being placed at the side of the instrument.
In the morning, the principal work consists in making what are termed
the “reductions” to the observations of the previous night. These
reductions are the corrections requisite for the slight instrumental
inaccuracy, for the refraction of the atmosphere, and for the known
constant error of the observer. When, therefore, a bright winter’s
night has occurred, the work on the following morning is usually very
heavy. At noon the sun’s time of transit is taken, and at one o’clock
the “ball” is dropped, by means of which the various vessels in the
Docks and in the Thames set their chronometers, or ascertain their
rate. In addition to this, the time is sent by electricity to Deal and
one or two other seaports, in order that every vessel may be able to
know the accurate time, if within sight of those places.
Not the least interesting portion of the observatory is the chronometer
room. For a very small charge, manufacturers or owners may have their
chronometers rated at Greenwich, which is accomplished in the following
manner:
The chronometer is placed in the chronometer room, and compared with
the large electric clock in the room, this clock being kept in order
by the stars. Each day the chronometer is examined, and thus its rate
is ascertained in its then temperature. It is afterwards placed in a
sort of closet warmed by gas, a condition supposed to represent the
tropics, and it is there kept for a certain period, being tested each
day as before. This change of temperature is found to produce very
little effect on the best instruments, which, when they have passed
the ordeal, are returned to the owners with their character ticketed
to them. Some hundred chronometers are often placed in this room; and
to compare them is a science, the “expert” by a glance discovering the
difference between the two instruments, whilst a novice would require
to mentally add or subtract, and thus slowly to arrive at the same
results.
As soon as it becomes dark enough to see stars by the aid of a
telescope, one of the staff commences his observations. These are
continued during the night; and a register is kept of each star,
planet, comet or moon, which is “doctored” in the morning by the
computers.
As all mortals are fallible, it is desirable to bring machinery into
use where possible, and this has been managed in connection with
astronomical observations. Instead of the computer registering by
judgment the time of a star’s transit over the various wires, he
strikes a small indicator, which, completing the electric circuit,
causes a pricker to fall and make a hole in a piece of paper that
is attached to a slowly revolving barrel. Each time the star passes
a wire, the pricker descends and leaves its mark; and the interval
between these marks being measured by scale, the mean time of transit
may be obtained.
There is usually a feeling of the sublime that comes over us when we
reflect upon the vast unexplored regions of space, or contemplate the
stellar world that shines upon us. The magnitude and grandeur of some
of the planets in the solar system strike us with a feeling of awe
and wonder, while we are puzzled at the mysteries attending comets,
double stars, nebulæ, etc. No such feelings or sentiments, however,
are allowed to enter into the constitution or mind of an observer at
Greenwich. Saturn, the glorious ringed planet, with its galaxy of
moons, is simply “Saturn, Right Ascension 10 hours 8 min. 12 sec.,
North declination 16° 12´ 2´´.” Anything appertaining to the physical
constitution, the probable cause of the ring, or the object of so
grand an orb, does not come within the range of the observations at
Greenwich, which are limited to bare matter-of-fact business work.
The southern portion of the observatory ground is devoted to
the investigation of meteorological subjects, and is under the
superintendence of Mr. Glaisher, who is now well known as an aerial
voyager. It is here that an exact record is kept of the amount of
rain that daily falls, of the direction and force of the wind, of
the magnetic changes, of the temperature, amount of ozone, etc.--all
matters which may, and probably will, lead us eventually to the
discovery of some laws connected with the states of weather, and enable
us to predict what may be expected from day to day. Whilst we are now
able to calculate to a few seconds, and for years in advance, the
instant when an eclipse may occur, and to explain the causes of the
various planetary movements, yet we are in a sad state of ignorance
as regards the causes of hurricanes, thunder-storms, continued rains
and droughts; and thus we find that all the would-be prophets who
from time to time spring up and oracularly announce a coming frost
or fine weather, or the reverse, are perpetually meeting with most
signal failures, which, however, does not deter future adventurers from
attempting to gain a cheap temporary renown by trying their luck at a
prophecy.
The perpetual accumulation of facts at Greenwich, whether these be of
an astronomical nature, or appertaining to the air we breathe and its
subtle changes, is a proceeding that must eventually lead us on to a
correct knowledge of the laws which govern these matters, and also keep
us acquainted with any variations that may be occurring in the elements
that surround us.
The order and quietness necessary in such calculations as those carried
on at Greenwich prevent it from being a “show” establishment, and
hence visitors are not admitted except on special business. Then,
however, every aid and assistance are offered to the student and
inquirer; the use of books and instruments is freely given, and such
information supplied as the little spare time of those belonging to the
establishment enables them to afford. Thus a visit to or a period of
study at Greenwich Observatory will amply repay those who wish to gain
the latest and most accurate information on astronomical subjects, or
to practise themselves at the adjustments and use of the instruments;
and to those who have not such opportunity, we offer this slight sketch.
[_Chambers’ Journal._
Pinions.
Well made as to truth of centring, of division, of form of leaves, and
polish, are, as the trade well knows, of vital importance to the value
of the time-piece.
The making and finishing is one of the most troublesome, as well
as most expensive of all the processes in watch work. The nature
of the material renders it difficult as it approaches so nearly in
hardness to the tools used in cutting. In the ordinary Yankee clock,
the _lantern pinion_ has entirely superseded the solid leaf, which
substitution was the greatest element of success in their cheap
construction. The lantern pinion is really a nearer approximation to
the required anti-frictional form than a majority of cut pinions in
ordinary clocks. In the process of manufacture of the cut variety, the
first consideration is the quality of the steel to be used. For this
purpose it should be carefully selected by trial, thus ascertaining
its fineness, uniformity, softness when annealed, together with its
capacity for taking a good temper, with the least amount of springing
during the hardening process. Very few pinions are cut from the solid
piece--the drawn pinion wire being quite good enough, when milled and
finished, for the ordinary run of watch work.
The steel wire having been selected, the first process is to cut it
up in lengths a trifle larger than the required pinion. The separated
pieces are then centred with care, and having been placed in a lathe,
the staff and pivot are turned up to nearly the required gauge, leaving
a portion of the whole piece the full size for the leaves. They are
now taken to the milling tool to have the proper form given to the
leaves. As this form is of the highest importance, it may be as well to
give here the reasons. Supposing a wheel of 60 teeth, depthing into a
pinion of 8 leaves, it can readily be seen that the arc of the motion
of the wheel tooth is of greater radius than that of the leaf of the
pinion, and it follows that if the teeth and the leaves are made in
taper form with straight sections, there must occur a sliding motion
on the surfaces of both--the power thus absorbed being totally wasted;
but if we curve the surfaces we may approach a form so nearly perfect
that the wheel teeth, being motors, really roll on the leaves, avoiding
almost entirely the friction caused by sliding; the necessity for this
curvature becoming greater the more the wheel exceeds the pinion in
diameter. This curve, which has been demonstrated by very profound
mathematical researches, is the “epicycloidal;” theoretically it should
give no more sliding motion than the surfaces of two plain wheels
revolving on each other. To obtain this perfect form, very great pains
have been taken and expenses incurred, especially by the makers of the
best time-keepers.
In the American factories the cutters are very elaborately made,
the section being an object of great solicitude--it being an exact
counterpart of the space between any two leaves, and also of one-half
the top of the leaf from the curvature to the point, so that in
milling, the space made by the cutter is its shape, leaving the leaf
of the proper form. Generally the pinion passes under two cutters; the
first to strike down the rough stock, the other to dress it to size and
shape, with a light cut. The care and skill required to make these is
certainly very great, and it is a proof of the wonderful ingenuity of
man that they are made so perfect as to shape and cutting power.