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_Dr. Rittenhouse’s Chronometer._
The construction of this Time-piece is thus described by Mr. Henry
Voight, chief coiner in the Mint, heretofore an eminent clock and
watch maker in Philadelphia; an artist of great ingenuity, and well
known for the excellence of his workmanship. The description is
given in Mr. Voight’s own words.
“The Clock which Dr. Rittenhouse made use of in his Observatory was a
construction of his own. It had but three wheels in its movement, of
high numbers. Only one pinion, without a wheel, driven by the main
wheel; whose axis goes through the front plate, that carried the
dial-work; and this wheel[A43] has a perpetual rochet.[A44] The seconds
are eccentric, as in the common clocks.
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Footnote A43:
“The main-wheel, which is fixed on the barrel on which the cat-gut
runs.” _Mr. Voight._
Footnote A44:
“A perpetual rochet is a spring lying between the main-wheel, and a
plate which is so high in diameter as to be nearly of a height with
the bottom of the main-wheel teeth, and is cut with fine teeth all
round, in the shape of a fine saw. A click on an axis is fixed between
the two frame-plates, with a weak spring that forces this click into
the fine saw-teeth, which keeps the plate from moving backwards when
the clock is winding up. This fine rochet-wheel is fixed on the
barrel-arbour or axis, the same as the main-wheel. The barrel-rochet
comes close against the plate of the fine rocket, which has a click
screwed on the front, corresponding with the barrel-rochet, and a
spring above that rochet’s click, which forces that click into the
barrel-rochet’s teeth: it is this that makes the clattering noise,
which is heard when a clock is winding up: There is a middling strong
spring placed between two arms of the cross of the main-wheel, bent
like the space of the two arms between which it acts; and this spring
is as broad as the thickness of the cross-arms. One end of that spring
is fastened to the inside of the fine rochet-plate: the other end lies
on the other cross-arm, and acts on that like a gun-lock mainspring on
the cock-tumbler. When the clock or time is set a going, and the
maintaining power or weight of the fusee or barrel, this power will
raise that spring so far as to resist the maintaining power, and
becomes stationary as long as the time-piece is going; and when it is
wound up, this spring in the main-wheel cross will expand itself,
press on the cross-arm, and force that wheel forward, with nearly the
same power as the maintaining power would give: the click for the
fine-teethed rochet falls into one of those fine teeth, and keeps that
rochet steady, without having the least motion, as long as the
winding-up of the clock continues; and by this means a time-piece can
lose no time in winding up: hence it is called a perpetual rochet;
which requires the most accurate workmanship, in its construction.”
_Mr. Voight._
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“The pallet-wheel moves outside of the back-plate, and the pallets are
fastened to the rod of the pendulum, which has double suspending springs
fixed in a cross-bar, to which the rod is rivetted in the middle. These
springs are suspended as in common; but they are not so long as in
general, and have only one-and-an-half inch free action, which keeps the
pendulum very steady in its vibrations.
“On the rod of the pendulum there is fixed a glass tube, of the
thickness of a strong thermometer-tube, and is in the whole as long as
the rod: but it is bent, about one-third upwards; like a barometer, but
longer; and upon that end, on the top, the tube is as wide again as it
is below, for about one-and-an-half inch in length: the other two-thirds
of the length is filled with spirits of wine; and at this end, the tube
is hermetically sealed. The shorter part is filled with mercury, so high
as to fill the widest part of it, about half an inch, and is not sealed
but remains open. The bend is close together, and there is no more space
between the tubes than three-eighths of an inch.
“This tube is fastened to the pendulum-rod with common sewing-thread,
and rests upon two pins fixed in the bob of the pendulum, as high up as
possible. The bob has no slide, but is immoveable; and the regulation of
the pendulum is performed by adding to, or diminishing the mercury, in
the part where the tube is widest.”
In addition to the foregoing description of the mechanism of this
Time-piece, obligingly furnished to the Writer of these Memoirs by
Mr. Voight, he has been likewise favoured by Robert Patterson, Esq.
director of the mint, with the following account of the same
extremely accurate instrument, which will greatly assist the reader
in understanding the principles on which it is constructed.
“In the Astronomical Clock made by Dr. Rittenhouse, and now in the Hall
of the Philosophical Society, I do not know,” says Mr. Patterson, “that
there is any thing peculiar, which requires mentioning, except the
pendulum; especially the apparatus for counteracting the effects of
change of temperature.
“For this purpose, there is fastened on the pendulum-rod (which is of
iron or steel) a glass tube of about thirty-six inches long; bent in the
middle into two parallel branches, at the distance of about an inch from
each other; the bend being placed downwards, immediately above the bob
of the pendulum. The tube is open at one end, and close at the other:
the arm which is close at top is filled, within about two inches of the
lower end or bend, with alcohol, and the rest of the tube, within about
one half of an inch of the upper extremity or open end, with mercury; a
few inches of the tube, at this extremity, being about twice the width
of the rest of the tube.
“Now, when the heat of the air encreases, it will expand the
pendulum-rod; and would thus lower the centre of oscillation, and cause
the clock to go slower: but this effect is completely counteracted, by
the expansion of the alcohol chiefly, and of the mercury in part; which
equally raises the centre of oscillation; and thus preserves an equable
motion in all the variable temperatures of the atmosphere.”
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_Description of an Hygrometer; first contrived and used by Dr.
Rittenhouse, about the year 1782._[A45]
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Footnote A45:
This description is drawn up from two separate accounts of the
instrument, with which the Writer of these Memoirs was obligingly
furnished, in writing, by Robert Patterson and the late David
Rittenhouse Waters, Esquires, of Philadelphia. Mr. Patterson mentions,
that he recollects his having seen the Hygrometer so described, in Dr.
Rittenhouse’s Observatory, about thirty years ago.
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The essential part of this Hygrometer consists of two very thin strips
of wood, about a foot long and half an inch broad, glued together, in
such a manner that the grain or fibres of the one shall be at right
angles with the other; so that when this compound strip was placed in
erect position, the grain of one of the pieces of wood would have a
vertical, and that of the other an horizontal position. One end of this
simply constructed instrument is to be made fast to a wall, or plane
board, with the edge outward, and the other end is to be at liberty to
move.
Then, as moisture has little or no effect on the length of a piece of
wood, or in the direction of its fibres, but a very sensible one on its
breadth, or transverse direction, especially when thin, it follows, that
on any increase of moisture in the air, this Hygrometer becomes bent
into a curve, convex on the side of the transverse fibres; and _vice
versâ_. The degrees, from the greatest dryness to the greatest moisture,
are to be marked on a curve drawn on the board or wall, described by the
motion of the free end of the Hygrometer; and an index, attached to the
moving end of it, will point out, on this graduated arch, the existing
state of the atmosphere at the moment, in relation to its condition of
moisture or dryness: The relative degree of either, on the smallest
change from the one to the other, will be indicated with much precision;
and probably, with much more uniformity and truth, in the results of
long-continued observations, than can be attained to by the use of
Hygrometers constructed of metal, or any other substance than wood.[A46]
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Footnote A46:
The second volume of the Transactions of the American Philosophical
Society contains a letter, written on the 13th of November, 1780, by
Dr. Benjamin Franklin, then in France, to Mr. Nairne, of London: but
it was not communicated to the Society, until January, 1786.
In that letter, Dr. Franklin suggests to Mr. Nairne (an eminent
optician, and mathematical instrument maker,) the idea of an
Hygrometer made of wood; in preference to metalline instruments, for
the purpose of discovering “the different degrees of humidity in the
air of different countries;”—an idea which occurred to the Doctor, in
consequence of a casual circumstance, mentioned in his letter.
Dr. Franklin supposed “a quick sensibility of the instrument, to be
rather a disadvantage” to it; “since,” says he, “to draw the desired
conclusions from it, a constant and frequent observation day and
night, in each country—when the design is, to discover the different
degrees of humidity in the air of different countries—will be
necessary for a year or years, and the mean of each different set of
observations is to be found and determined.”—“For these reasons,”
continues the Doctor, “I apprehend that a substance which, though
capable of being distended by moisture and contracted by dryness, is
so slow in receiving and parting with its humidity that the frequent
changes in the atmosphere affect it sensibly, and which therefore
should, gradually, take nearly the medium of all those changes and
preserve it constantly, would be the most proper substance, of which
to make an Hygrometer:”—and he believes _good mahogany wood_ to be
that substance. In the concluding part of this letter, Dr. Franklin
says to his correspondent: “I would beg leave to recommend to you—that
you would take a number of pieces of the closest and finest grained
mahogany that you can meet with; plane them to the thinness of about a
line, and the width of about two inches across the grain, and fix each
of the pieces in some instrument that you can contrive, which will
permit them to contract and dilate, and will shew, in sensible
degrees, by a moveable hand upon a marked scale, the otherwise less
sensible quantities of such contraction and dilatation.”
Hence it appears, that Franklin and Rittenhouse conceived an idea of
the same kind, nearly at the same time: but that the latter carried
his invention into practice, three or four years before the theory of
the former, founded on similar principles, had been announced to the
American public, or, as it is believed, was made known to any other
person than Mr. Nairne. W. B.
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_Astronomical Observations, made in the years 1776, 1777 and 1778, at
Philadelphia, by the Rev. Dr. W. Smith, and David Rittenhouse, John
Lukens, and Owen Biddle, Esquires: copied from a manuscript account of
those Observations, drawn up by Dr. Smith; never before published._