the sand was allowed to run out through a variable orifice below, could
be made to descend with any desired velocity and yet with uniformity.
In addition, by these means an unlimited power could be brought to
bear, depending on the size of the weight. Previously it was proposed
to use water, and compensate for the decrease in flow, as the column
shortened, by a conical vessel; but it was soon perceived that as
each drop of water escaped from the funnel-shaped vessel, only a
corresponding weight would be brought into play. This is not the case
with sand, for in this instance every grain that passes out causes the
whole weight that is supported by the column to come into action. In
the former instance a movement consisting of a series of periods of
rest and periods of motion occurs, because power has to accumulate by
floating weight lagging behind the descending water, and then suddenly
overtaking it. In the latter case, on the contrary, there is a regular
descent, all minor resistances in the slide being overcome by the
steady application of the whole mass of the weight.
When these advantages in the flow of sand were ascertained, all the
other prime movers were abandoned. Mercury-clocks, on the principle of
the hydrostatic paradox, air-clocks, &c., in great variety, had been
constructed.
[Illustration: Fig. 34. The Sand-Clock.]
The sand-clock consisted of a tube _a_ (Fig. 34), eighteen inches
long and one and a half in diameter, nearly filled with sand that had
been raised to a bright red heat and sifted. Upon the top of the sand
a leaden weight _b_ was placed. At the bottom of the tube a peculiar
stopcock, seen at (2) enlarged, regulated the flow, the amount passing
depending on the size of the aperture _d_. This stopcock consisted
of two thin plates, fixed at one end and free at the other. The one
marked _e_ is the adjusting lever, and its aperture moves past that in
the plate _g_. The lever _f_ serves to turn the sand off altogether,
without disturbing the size of the other aperture, which, once set
to the moon’s rate, varies but slightly in short times. A movable
cover _h_, perforated to allow the cord _i_ to pass through, closed
the top, while the vessel _k_ retained the escaped sand, which at
suitable times was returned into the tube _a_, the weight _b_ being
temporarily lifted out. From the clock the cord _i_ communicated motion
to the frictionless slide, as shown in Fig. 33. This cord should be as
inelastic as possible, consistent with pliability, and well waxed.
One who has not investigated the matter would naturally suppose that
the flow of sand in such a long tube would be much quicker when the
tube was full than when nearly empty, and that certainly that result
would occur when a heavy weight was put on the shifting mass. But in
neither case have I been able to detect the slightest variation, for,
although by shaking the tube a diminution of the space occupied by the
sand may be caused, yet no increase of weight tried could accomplish
the same reduction. These peculiarities seem to result from the sand
arching as it were across the vessel, like shot in a narrow tube, and
only yielding when the under supports are removed. In blasting, a heavy
charge of gunpowder can be retained at the bottom of a hole, and made
to split large masses of rock, by filling the rest of the hole with dry
sand.
I believe that no prime mover is more suitable than a sand-clock
for purposes where steady motion and a large amount of power are
demanded. The simplicity, for instance, of a heliostat on this plan,
the large size it might assume, and its small cost, would be great
recommendations. In these respects its advantages over wheelwork are
very apparent. The precision with which such a sand-clock goes may be
appreciated when it is stated, that under a power of 300 a lunar crater
can be kept bisected for many times the period required to photograph
it. To secure the greatest accuracy in the rate of a sand-clock, some
precautions must be taken. The tube should be free from dents, of
uniform diameter, and very smooth or polished inside. Water must not be
permitted to find access to the sand, and hygrometric varieties of that
substance should be avoided, or their salts washed out. The sand should
be burned to destroy organic matter, and so sifted as to retain grains
nearly equal in size. The weight, which may be of lead, must be turned
so as to go easily down the tube, and must be covered with writing
paper or some other hard and smooth material, to avoid the proneness to
adhesion of sand. A long bottle filled with mercury answers well as a
substitute.
I have used in such clocks certain metallic preparations: Fine shot, on
account of its equality of size, might do for a very large clock with
a considerable opening below, but is unsuitable for a tube of the size
stated above. There is, however, a method by which lead can be reduced
to a divided condition, like fine gunpowder, when it may replace the
sand. If that metal is melted with a little antimony, and while cooling
is shaken in a box containing some plumbago, it breaks up at the
instant of solidifying into a fine powder, which is about five times
as heavy as sand. If after being sifted to select the grains of proper
size, it is allowed to run through a small hole, the flow is seen to be
entirely different from that of sand, looking as if a wire or solid rod
were descending, and not an aggregation of particles. It is probable,
therefore, that it would do better than sand for this purpose. I have
not, however, given it a fair trial, because just at the time when the
experiments with the sand-clock had reached this point, I determined to
try a clepsydra as a prime mover.
The reason which led to this change was that it was observed on a
certain occasion when the atmosphere was steady, that the photographs
did not correspond in sharpness, being in fact no better than on other
nights when there was a considerable flickering motion in the air. A
further investigation showed that in these columns of sand there is
apt to be a minute vibrating movement. At the plate-holder above this
is converted into a series of arrests and advances. On some occasions,
however, these slight deviations from continuous motion are entirely
absent, and generally, indeed, they cannot be seen, if the parts of the
image seem to vibrate on account of currents in the air. By the aid
of the microscopic exaggeration described on a former page--which was
subsequently put in practice--they may be observed easily, if present.
When the negative produced at the focus of the great mirror is intended
to be enlarged to two feet or more in size, these movements injure it
sensibly. A variety of expedients was resorted to in order to avoid
them, but none proved on all occasions successful.
It is obvious that in a water-clock, where the mobility of the fluid
is so much greater than that of solid grains, this difficulty would
not arise. The following contrivance in which the fault of the ordinary
clepsydra, in varying rate of flow as the column shortens, is avoided,
was next made. With it the best results are attainable, and it seems to
be practically perfect.
[Illustration: Fig. 35. The Clepsydra.]
It consists of a cylinder _a_, in which a piston _b_ moves watertight.
At the top of the piston rod is a leaden five-pound weight _c_, from
which the cord _i_ goes to the sliding plateholder _g_. The lower end
of the cylinder terminates in a stopcock _d_, the handle of which
carries a strong index rod _e_, moving on a divided arc. At _f_ a tube
with a stopcock is attached. Below, a vessel _h_ receives the waste
fluid.
In using the clepsydra the stopcock of _f_ is opened, and the piston
being pulled upwards, the cylinder fills with water from _h_. The
stopcock is then closed, and if _d_ also is shut, the weight will
remain motionless. The string _i_ is next connected with the slide, and
the telescope turned on the moon. As soon as the slide is adjusted in
angular position (page 36) the stopcock _d_ is opened, until the weight
_c_ moves downwards, at a rate that matches the moon’s apparent motion.
In order to facilitate the rating of the clepsydra, the index rod
_e_ is pressed by a spring _k_ (2), against an excentric _l_. As the
excentric is turned round, the stopcock _d_ is of course opened,
with great precision and delicacy. The plug of this stopcock (3) is
not perforated by a round hole, but has a slit. This causes equal
movements in the rod _e_, to produce equal changes in the flow. The
rating requires consequently only a few moments.
The object of the side tube _f_ is to avoid disturbing _d_ when it
becomes necessary to refill the cylinder, for when it is once opened
to the right degree, it hardly requires to be touched again during a
night’s work. In order to arrest the downward motion of the piston at
any point, a clamp screws on the piston rod, and can be brought into
contact with the cylinder head, as in the figure.
That this instrument should operate in the best manner, it is essential
to have the interior of the brass cylinder polished from end to end,
and of uniform diameter. If any irregularity should be perceived in the
rate of going, it can be cured completely by taking out the piston,
impregnating its leather stuffing with fine rotten stone and oil, and
then rubbing it up and down for five minutes in the cylinder, so as to
restore the polish. The piston and cylinder must of course be wiped,
and regreased with a mixture of beeswax and olive oil (equal parts)
after such an operation. In replacing the piston, the cylinder must be
first filled with water, to avoid the presence of air, which would act
as a spring.
Although it may be objected that this contrivance seems to be very
troublesome to use, yet that is not the case in practice. Even if it
were, it so far surpasses any prime mover that I have seen, where the
utmost accuracy is needed, that it would be well worth employing.
c. _The Sun Camera._
In taking photographs of the sun with the full aperture of this
telescope, no driving mechanism is necessary. On the contrary, the
difficulty is rather to arrange the apparatus so that an exposure
short enough may be given to the sensitive plate, and solarization of
the picture avoided. It is not desirable to reduce the aperture, for
then the separating power is lessened. The time required to obtain a
negative is a very small fraction of a second, for the wavy appearance
produced by atmospheric disturbance is not unfrequently observed
sharply defined in the photograph, though these aerial motions are so
rapid that they can scarcely be counted. Some kind of shutter that can
admit and cut off the solar image with great quickness is therefore
necessary.
[Illustration: Fig. 36. The Spring Shutter.]
In front of an ordinary camera _a_, Fig. 36, attached to the eyepiece
holder of the telescope, and from which the lenses have been removed,
a spring shutter is fixed. It consists of a quadrant of thin wood _b_,
fastened by its right angle to one corner of the camera. Over the
hole in this quadrant a plate of tin _d_ can be adjusted, and held
in position by a screw moving in a slot so as to reduce the hole if
desired to a mere slit. It may vary from 1-1/2 inch to less than 1/50
of an inch. The quadrant is drawn downwards by an India-rubber spring
_g_, 1 inch wide, 1/8 of an inch thick, and 8 inches long. This spring
is stretched when in action to about 12 inches, and when released draws
the slit past the aperture _c_ in the camera. Two nicks in the edge of
the quadrant serve with the assistance of a pin _e_, which can easily
be drawn out by a lever (not shown in the cut), to confine the slit
either opposite to or above _c_. A catch at _f_ prevents the shutter
recoiling. The sensitive plate is put inside the box as usual in a
plate-holder. When a photograph is taken, the spring shutter is drawn
up so that the lower nick in the edge of the quadrant is entered by the
pin _e_, and the inside of the camera obscured. The front slide of the
plateholder is then removed in the usual manner, and the solar image
being brought into proper position by the aid of the telescope finder,
the trigger retaining _e_ is touched, the shutter flies past _c_, and
the sensitive plate may then be removed to be developed.
To avoid the very short exposure needed when a silvered mirror of 188
square inches of surface is used, I have taken many solar photographs
with an unsilvered mirror, which only reflects according to Bouguer
2-1/2 per cent. of the light falling upon it, and should permit an
exposure 37 times as long as the silvered mirror. This is the first
time that a plain glass mirror has been used for such a purpose,
although Sir John Herschel suggested it for observation many years ago.
But eventually this application of the unsilvered mirror had to be
abandoned. It has, it is true, the advantage of reducing the light and
heat, but I found that the moment the glass was exposed to the Sun, it
commenced to change in figure, and alter in focal length. This latter
difficulty, which sometimes amounts to half an inch, renders it well
nigh impossible to find the focal plane, and retain it while taking
out the ground glass, and putting in the sensitive plate. If the glass
were supported by a ring around the edge, and the back left more freely
exposed to the air, the difficulty would be lessened but not avoided,
for a glass mirror can be raised to 120° F. on a hot day by putting
it in the sunshine, though only resting on a few points. Other means
of reducing the light and heat, depending on the same principle, can
however be used. By replacing the silvered diagonal mirror with a black
glass or plain unsilvered surface, as suggested by Nasmyth, the trouble
sensibly disappears.
I have in this way secured not only maculæ and their penumbræ, but
also have obtained faculæ almost invisible to observation. On some
occasions, too, the precipitate-like or minute flocculent appearance on
the Sun’s disk was perceptible.
It seems, however, that the best means of acquiring fine results with
solar photography, would be to use the telescope as a Cassegrainian,
and produce an image so much enlarged, that the exposure would not have
to be conducted with such rapidity. Magnifying the image by an eyepiece
would in a general way have the same result, but in that case the
photographic advantages of the reflector would be lost, and it would be
no better than an achromatic.
§4. THE OBSERVATORY.
This section is divided into _a_, The Building; _b_, The Dome; and _c_,
The Observer’s Chair.
a. _The Building._
The Observatory is on the top of a hill, 225 feet above low water mark,
and is in Latitude 40° 59′ 25″ north, and Longitude 73° 52′ 25″ west
from Greenwich, according to the determinations of the Coast Survey.
It is near the village of Hastings-upon-Hudson, and is about 20 miles
north of the city of New York. The surrounding country on the banks
of the North River is occupied by country seats, on the slopes and
summits of ridges of low hills, and no offensive manufactories vitiate
the atmosphere with smoke. Our grounds are sufficiently extensive to
exclude the near passages of vehicles, and to avoid tremor and other
annoyances.
[Illustration: Fig. 37. Dr. Draper’s Observatory.]
An uninterrupted horizon is commanded in every direction, except
where trees near the dwelling house cut off a few degrees toward the
southwest. The advantages of the location are very great, and often
when the valleys round are filled with foggy exhalations, there is a
clear sky over the Observatory, the mist flowing down like a great
stream, and losing itself in the chasm through which the Hudson here
passes.
The foundation and lower story of the building are excavated out of the
solid granite, which appears at the edge of the hill. This arrangement
was intended to keep the lower story cool, and avoid, in the case of
the metal reflector, sudden changes of temperature. The eastern side
of the lower story, however, projects over the brow of the hill, and
is therefore freely exposed to the air, furnishing, when desired, both
access and thorough ventilation through the door. The second story or
superstructure is of wood, lined inside with boards like the story
below. They serve to inclose in both cases a non-conducting sheet of
air.
The inside dimensions of both stories taken together are 17-1/2 feet
square, and 22 feet high, to the apex of the dome. This space is
unnecessarily large for the telescope, which only requires a cylinder
13 feet in diameter and 13 feet high. A general idea of the internal
arrangement is gained from Fig. 28. In Fig. 38, _a a′_ is the floor of
the gallery, _b b′ b″_ the circular aperture in which the telescope _c
c′_ turns. The staircase is indicated by _d_. The Enlarger, §6, rests
on the shelf _e_, the heliostat being outside at _f_. The door going
into the photographic room is at _g_, _h h′_ are tables, _i_ the water
tank, _k_ the tap and sink, _l_ the stove, _m_ a heliostat shelf, _n_
the door, _o_ the window.
[Illustration: Fig. 38. Plan of Observatory (upper floor).]
The building is kept ventilated by opening the door in the lower part,
and the dome shutter, seen in Fig. 37, for some time before using the
instrument. On a summer day the upper parts, and especially those close
under the dome, become without this precaution very hot, and this
occurred even before the tin roof was painted. Bright tinplate seems
not to be able to reflect by any means all the heat that falls upon
it, but will become so warm in July that rosin will melt on it, and
insects which have lighted in a few moments dry up, and soon become
pulverizable. A knowledge of these facts led to the abandonment of
wooden sheathing under the tin, for without it when night comes on the
accumulated heat radiates away rapidly, and ceases to cause aerial
currents near the telescope.
The interior of the building is painted and wainscoted, and the roof is
ornamented partly in blue and oak, and partly with panels of tulip-tree
wood.
There are only two windows, and they are near the southern angles
of the roof. While they admit sunshine on some occasions, they can
on others be closed, and the interior be reduced to darkness. In
the southeast corner a small opening _e_ may allow a solar beam
three inches in diameter to come in from a heliostat outside. The
greatest facilities are thus presented for optical and photographical
experiments, for in the latter case the whole room can be used as a
camera obscura.
b. _The Dome._
The roof of the observatory is 20 feet square. The angles are filled in
solid, and a circular space 15 feet in diameter is left to be covered
by the revolving dome. Although such a construction is architecturally
weak and liable to lose its level, yet the great advantages of
having the building below square, and the usefulness of the corners,
determined its adoption, the disadvantages being overcome by a very
light dome.
The dome is 16 feet in outside diameter, and rises to a height of 5
feet above its base. It is, therefore, much flatter than usual, in
fact, might have been absolutely flat, with this method of mounting.
It would then have been liable, however, to be crushed in by the deep
winter snows.
It consists of 32 ribs, arcs of a circle, uniting at a common centre
above. Each one is formed of two pieces of thin whitewood, _b_,
Fig. 39, fastened side by side, with the best arrangements of the grain
for strength. They are three inches wide and one inch thick at the
lower end, and taper gradually to 2-1/2 by 1.
[Illustration: Fig. 39. Joints in Tin of Dome.]
Over these ribs tinplate is laid in triangular strips or gores, about
18 inches wide at the base, and 10 feet long. Where the adjacent
triangles of tin _a a′_ meet, they are not soldered, but are bent
together. This allows a certain amount of contraction and expansion,
and is water-proof. It strengthens the roof so much, that if the ribs
below were taken away, this corrugated though thin dome would probably
sustain itself. The tin is fastened to the dome ribs _b_ by extra
pieces _c_ inserted in the joint and doubled with the other parts,
while below they are nailed to the ribs. In the figure the tin is
represented very much thicker than it is in reality.
This dome, although it has 250 square feet of surface, only weighs 250
pounds. That at the Cambridge (Massachusetts) Observatory, 29-1/2 feet
in diameter, weighs 28,000 pounds.
The slit or opening is much shorter than usual, only extending half way
from the base towards the summit. It is in reality an inclined window,
2-1/2 feet wide at the bottom, 1-1/2 wide at the top, and 4 feet long.
It is closed by a single shutter, as seen in Fig. 37, and this when
opened is sustained in position by an iron rod furnished with a hinge
at one end and a hook at the other.
The principal peculiarity of the dome, the means by which it is
rotated, remains to be described. Usually in such structures rollers
or cannon balls are placed at intervals under the edge, and by means
of rack work, a motion of revolution is slowly accomplished. Here,
on the contrary, the whole dome _b b′ b″_ (Fig. 40) is supported on
an arch _h h′ h″_, carrying an axis a at its centre, around which a
slight direct force, a pull with a single finger, will cause movement,
and by a sudden push even a quarter of an entire revolution may be
accomplished. It is desirable, however, to let it rest on the edge
_b b″_, when not in use. At _c_ there is an iron catch on the arch, by
which the lever _e_, that raises the dome, is held down. The fulcrum is
at _d_. The lever is hinged near _c_, so that when by being depressed
it should have come in the way of the telescope below, the lower half
_g_ can be pushed up, the part from _c_ toward _d_ still holding the
dome supported.
[Illustration: Fig. 40. The Dome Arch.]
The arch can be set across the observatory in any direction, north and
south, east and west, or at any intermediate position, because the
abutments where the ends rest, are formed by a ring _l l′ l″_, fastened
round the circular aperture, through the stationary part of the roof.
When the telescope is not in use, and the dome is let down, so that
there is no longer an interval of a quarter of an inch between it and
the rest of the roof, it is confined inside by four clamps and wedges.
Otherwise, owing to its lightness, it would be liable to be blown away.
These clamps _a_, Fig. 41, are three sides of a square, made of iron
one inch square. They catch above by a point in the wooden basis-circle
of the dome _b_, and below are tightened by the wedge _c_.
[Illustration: Fig. 41. A Dome Clamp.]
When the dome is raised it is prevented from moving laterally and
sliding off by three rollers, one of which is seen at _f_, Fig. 40.
These catch against its inner edge, and only allow slight play. At
first it was thought necessary to have a subsidiary half arch at right
angles to the other to hold it up, but that is now removed.
All the parts work very satisfactorily, and owing to the care taken to
get the roof-circle and basis-circle flat and level, no leakage takes
place at the joint, and even snow driven by high winds is unable to
enter.
c. _The Observer’s Chair._
This is not a chair in the common acceptation of the word, but is
rather a movable platform three feet square, capable of carrying two
or more persons round the observatory, and maintaining them in an
invariable position with regard to the telescope eyepiece.
[Illustration: Fig. 42. The Observer’s Chair.]
Its general arrangement is better comprehended from the sketch,
Fig. 42, than from a labored description. Below, it runs on a pair of
wheels _a_ (one only is visible) 9 inches in diameter, whose axles
point to the centre of the circle upon which they run. They are
prevented from shifting outwards by a wooden railroad _b_, _b′_, and
inwards by the paling _l_, _l′_. Above, the chair moves on a pair of
small rollers _c_, which press against a circular strip or track _d_,
_d′_, nailed around the lower edge of the dome opening. Access to the
platform is gained by the steps _e_, _e′_. Attached to the railing of
this platform, and near it on the telescope, are two tables (not shown
in the figure) for eyepieces, the sliding plateholder, &c.
§5. THE PHOTOGRAPHIC LABORATORY.
This section is divided into _a_, Description of the Apartment; and
_b_, Photographic Processes.
a. _Description of the Apartment._
The room in which the photographical operations are carried on, adjoins
and connects with the observatory on the southeast, as is shown in
Figs. 28 and 38. It is 9 by 10 feet inside, and is supplied with
shelves and tables running nearly all the way round, which have upon
them the principal chemical reagents. It is furnished, too, with an
opening to admit, from a heliostat outside, a solar beam of any size,
up to three inches in diameter.