30.5 
HOROLOG Y. 
finks down as tlie furface of the liquid falls, it is evident 
that the velocity of its efflux will be uniform. If the 
height of the veflel DE be therefore divided into equal 
parts, thefe divifions will indicate equal intervals of time. 
To render this clepfydra more curious, the branch A B 
might be concealed, by a frnall light figure made to float 
on the furface of the water in the veflel, and indicating 
the hour with a rod, or its finger, on a finall dial-plate. 
On the other hand, the water might be made to flow from 
any veflel whatever, through a fimilar fyphon, into another 
veflel of a pril'matic or cylindric form, from which might 
arife a frnall figure floating on the water, to indicate the 
hour above defcribed. 
In the fame Recreations, vol. ii. it is noted that father 
Timothy, a Barnabite, had given the clepfydra all the ex¬ 
cellence it was capable of, by conftrufting it fo as to make 
it go a month at one winding up, and to exhibit not only 
the hours on a dial-plate, but alfo the fun’s place, day of 
the month, and feftivals throughout the year. How thefe 
and fimilar particulars might be indicated, will be eafily 
apprehended from the following defcription : ABCJJ, 
Plate VI. fig. 28, is an oblong frame of wood, to the up¬ 
per part of which two cords, A a and B b, are fixed at 
their fuperior extremities, and at their inferior to the 
metallic arbor, a b, of the drum, E, which contains dif- 
tilled water; this water is confined in cells fo peculiarly 
conftrufted, that they regulate the velocity with which the 
drum fliall defcend by the force of gravity from the top 
to the bottom of the frame, and the ends of the arbor in¬ 
dicate the hours marked on the vertical plane of the frame 
during the time of defcent. An obferver, who knows not 
the nature of the interior cells of the drum, is furprifed 
to fee that its weight does not make it run down rapidly, 
’when mounted toYhe top of the frame by merely folding 
the firings round the arbor, there being apparently no 
mechanical impediment to the natural action of gravity. 
To explain how this phenomenon is produced, we mult 
refer to fig. 29, which is a feftion of the drum at right 
angles to its arbor; this circular plane we will fuppole to 
be fix inches, which is about the ufual iize, in diameter, 
and to reprefent the inner furface of either of the two ends 
of the drum, which may be made of any of the unoxyda- 
ble metals: then, if we conceive feven metallic partitions, 
F f, G g, H /;, I i, K $, L/, and M m, to be ciofely loldered 
to both ends of the drum, in the iloping direction indi¬ 
cated by the figure, where the black lines are equi-diftant 
tangents to the frnall dotted circle of an inch and half 
diameter at the points f, g, k, &c. it is evident, that any 
frnall quantity of water introduced into the drum would 
fall into two, or at moft three, of the lower Compartments, 
and would remain there until fome external force fhoukl 
alter the pofition of the drum, fuppofing in this cafe the 
cords tied faft to the arbor ; but we have laid that they 
are wound round the circumference of an arbor, that has 
a fenfible diameter, fuppofe one-eighth of an inch ; there¬ 
fore, they are removed one-fixteenth of an inch, or up¬ 
wards, if we take their thicknefs into the account, from 
the centre of the drum, which would alfo be its centre of 
gravity if it were empty, on which account it would, in 
that cafe, revolve to the left, in the direction F G H, down¬ 
wards, from the cord being at the remote fide of the cen¬ 
tre, as reprefented by N O; but conceive the water to be 
included now, and then it would be elevated to the right, 
till its weight became a counterpoife to the gravity of the 
heavier fide of the drum, in which fituation all motion 
would ceafe, and the drum would remain,.fufpended, in¬ 
deed, by the cords, but in a ftate of equiiibrio. Conceive 
again a frnall hole perforated in the partition preffed upon 
by the water near the circumference of the large circle, 
and alfo at the points F, G, H, I, K, L, M; and the con- 
fequence will be, that the water will firft force its way 
flowly through the perforation at K, from the more ele¬ 
vated to the lower compartment, which effect wiil diininilh 
its power as a counterpoife, and give fuch an advantage 
So the heavy fide, FGH, of the drum, confidered as 
Vol. X. No. 662. 
empty, as will occafion a frnall degree of motion towards 
the left, and ccnfequently carry the water once more to¬ 
wards the right; but now the water pafles through the 
perforation of the next partition alfo at I, and produces 
again the fame effect as has been defcribed wjtli refpeft 
to K, and will continue to do lb, at the fucceflive perfo¬ 
rations, till all the compartments have been filled and 
emptied by means of thefe perforations in fucceffion; which 
kind of motion of the drum, contrary to that of the wa¬ 
ter, it is now not difficult to conceive will be pretty regu¬ 
lar, if all the partitions are perforated exactly alike. The 
difference of the preffures of the water in cells, nearly full 
and nearly empty, wiil occafion fome little deviation from 
regularity; but thefe wiil be periodic, and muff be allowed 
for in the hour-divilions, which ought to be made by a 
companion of the fpaces fallen through with the time 
indicated by a clock or watch. About nine ounces of 
diffilled water will fuffice for a clepfydra of fix inches 
diameter and two inches depth; and the velocity of the 
fall may be limited either by varying the quantity of wa¬ 
ter, or by banging a final} metallic cup, F, to receive 
weights, by a cord wound in a direction contrary to the 
cords of fufpenfion, to aft as a counterpoife in aid of the 
water, if the fall be too rapid, or vice verja. 
It is abfolutely necefiary that the arbor Ihouid fit the 
central l'quare hole fo well as to prevent the efcape of wa¬ 
ter from the drum, otherwife the inffrument would con¬ 
tinue to gain velocity, till at length it would no longer 
afford a true indication of time. 
Sometimes a cord, cd, with a weight, P, is made to pafs- 
round a pulley.fixed to an arbor at the top of the frame, 
with a noofe palling over the axis near a, as is feen in the 
fame figure; which arbor, projecting through a dial-plate 
or face, turns round and carries a hand to indicate the 
hours like an ordinary clock; when this conftruftion is 
preferred, it is an inaifpenfable requifite tint the circum* 
ference of tlie pulley’s groove be exaftlv of the fame di- 
menfions as tlie fall of the drum in 12 or 24 hours, ac¬ 
cordingly as the dial is divided. 
This clepfydra, it is faid, goes fafter in fummer than in 
winter, which is owing to the drum being relatively Hea¬ 
vier in rarefied than in denfe air; we can hardly fuppofe 
that any alteration in the fluidity of the water, as for¬ 
merly fuppofed, would make any difference. The minute- 
hand and all'o the ftriking-part of a common clock might 
eafily be fuperadded to this clepfydra. 
We fhall conclude our account of thefe horological 
inftruments with detailing the conftruftion and aftion 
of a clepfydra, publiflied in the 44th volume of the 
Philofophical Tranfaftions by the Hon. Mr. Charles 
Hamilton. 
A B and C D, fig. 30, are two fimilar oblong veffels at¬ 
tached to a frame of wood, which may eafily be conceived 
to furround the interior mechanifm of the inffrument: 
ab and cd are two columns of wood fo floating in water, 
that their counterpoiles, F and G, juft keep their fuperior* 
ends equal with the furface of the water by means of'con- 
nefting chains palling over the pulley f, and another hid 
by the dial-plate; the former of thefe pulleys f, has a click 
which pufhes the ratchet on the barrel, i, when the counter¬ 
poife, F, falls, but flips eafily over the Hopes of the teeth 
when the faid counterpoife rifes ; the latter pulley has alfo 
a fimilar dick afting in like manner, with a fecond ratchet 
at the oppoflte end of barrel, i, which ratchet is alfo hid 
in the drawing; I’o that, whichever of the two counter - 
poifes fliall at any time be falling, the barrel, i, will move 
forwards in the fame direftion; and carry the minute- 
hand along with it on the dial-plate; the hour-hand goes 
round by means of dial-work, as in an ordinary, clock or 
watch, where a diminution of velocity is effected by two 
wheels and two pinions. The aftion is thus produced by 
means of five lyphons and two balances. 
The water enters with an unvaried influx, drawn from 
a relervoir, by a fyphon of finall bore, the longer leg of 
which is feen at I, into the middle of what may be called 
4 I a 
