503 
H 
HOROLOGY. 
II. Of CLEPSYDRA, or WATER-CLOCKS. ■ 
Though we have ranked thefe in the fecond clafs of 
Xime-meafurers, yet fome are of opinion that they might 
have been in ufe among the Egyptians and other eaftern 
nations even before fun-d'iais were invented; though the 
name of the original inventor is not handed down to us. 
The conftruftion has been varied in different ages and 
countries, according to the variation of the different modes 
of reckoning time; but one principle is the bafis of all the 
forms it has undergone, namely, the conftant dropping 
or running of water, through a fmall aperture, out of one 
veffel into another. At firft the indication of time was 
effected by marks correfponding to either the diminution 
of the fluid in the containing veffel during the time of 
emptying, or to the increafe of the fluid in the receiving 
veffel during its time of filling ; but it was foon found, 
that the efcape of the water was much more rapid out of 
the containing veffel when it was full than when it was 
nearly empty, owing to the difference of preffures-at dif¬ 
ferent heights of the furface; this irregularity in the 
dropping prefented an obftacle which required much in¬ 
genuity to correct. 
At any rate, we are well allured that the ancients had 
machines by which, through the help of water, they were 
.able to meafure time. The invention of them is by Vi¬ 
truvius afcribed to Ctefibius of Alexandria, who lived 
under Ptolemy Euergetes, or about the year 24.5 before 
the Chriftian era.- They were introduced at Rome by P. 
.Cornelius Scipio Nafica, in the year 595 after the build¬ 
ing of the city, or 159 years before the birth of Chrift. 
If under this name we underhand thole meafurers of time 
which were ufed in courts of juftice, the clepfydra is a 
Grecian invention, firft adopted at Rome under the third 
confuilhip of Pompey. The moft common kinds of thefe 
water-clocks all, however, correfponded in this, that the 
water iffued drop by drop through a hole of the veffel, 
and fell into another, in which a light body that floated 
marked the height of the water as it rofe, and by thefe 
•means the time that had elapled. They all had this fail¬ 
ing in common, that the water at firft flowed out rapidly, 
and afterwards more flowly, fo that they required much 
care and regulation. 
That ingenious machine, which we have at prefent un¬ 
der the name pf a water-clock, was invented in the laft 
century. The precife time feems to be uncertain; but, 
•had it been before the year 164.3, ICircher, who mentions 
all the then known machines of this kind, would in all 
probability have taken notice of it. It confifts of a cy¬ 
linder divided into feveral fmall cells, and fufpended by 
•a thread fixed to its axis, in a frame on which the hour- 
■diftances, found by trial, are-marked out. As the water 
■flaws from the one cell into the other, it changes very 
flowly the centre of gravity of the cylinder, and puts it 
in motion ; much like the quicklilver puppets invented 
by the Chinefie. This machine, with improvements, will 
be delcribed farther on. 
Thefe machines mull have been very fcarce in France 
in 1691 ; for Graverol at that time gave a figure and de¬ 
scription of the external parts of one, but promifed to 
give the internal conftruftion as loon as he thou Id become 
acquainted with it. This was the only one then in Nifmes. 
He fays, alfo, that they were invented a little before by an 
•Italian Jefuit, who refided at Bologna, but were brought 
to perfeftion by Taliaiffon, profeffor of law at Thouloufe, 
and a young clergyman named De L’ille. Alexander fays 
more than once that this machine was invented at Sens in 
Burgundy, in 1690, by Charles Vailly, a Benediftine of 
the brotherhood of St. Maur, and that he brought it to 
perfeftion by the afliftance of a pewterer there, named 
Regnard. This account is in fome meafhre confirmed by 
Ozanam; for he fays exprefsly, that the firft water-clocks 
were brought from Burgundy to Paris in 1693, and he 
-defcribes one which was made of tin at Sens. Vailly was 
born at Paris in 164.6, and died in 1726 ; he was cele¬ 
brated on account of his mathematical knowledge, though 
he is known by no works, as he burned aB his tnanu- 
fcvipts. Alexander, however, who was of the fame order, 
feems to have afcribed to his brother Benediftine an ho¬ 
nour to which he was not entitled: for Dominic Marti- 
nelli, an Italian of Spoletto, publiftied at Venice, in 1663, 
a treatife written exprefsly on thefe water-clocks, which 
Ozanam got tranflated into French, by one of his friends, 
and caufed to be printed with his additions. This tranf- 
lator fays, that water-clocks were known in France twenty 
years earlier than Ozanam had imagined. It appears, there¬ 
fore, that they were invented in Italy about the middle of 
the 17th century ; and that Vailly, perhaps, may have firft 
made them known in France. 
It may, perhaps, afford fome pleafure to thofe who are 
fond of the hiftory of the arts, to know that Salmon, an 
ingenious pewterer at Chartres in France, has given very 
full and ample directions how to conftruft and Ufe this 
machine. He is of opinion, that the invention is fcarcely 
a century old ; and that thele water-clocks, which are 
now common, were firft made for fale, and brought into 
ufe among the people in the country, by a pewterer at 
Sens in Burgundy. What this artift affirms, that they can 
be conftrufted of no metal fo eafily, fo accurately, and to 
'laft fo long, as of tin, is ftilbfound in practice to be true. 
The moft ancient’water-clock, then, of which we have 
any certain notice, was that of Ctefibius of Alexandria, 
the fon of a barber, who, about 245 years before Chrift, 
fpent much time in deviling mechanical contrivances for 
removing not only the obftacle common to all thefe ma¬ 
chines, but alio another equally formidable one, which 
arofe from the daily inequality of the Egyptian hours, as 
juft noticed in treating of Sun-dials. As one-twelfth part 
of the time elapled from fun-rife to fun-fetting of any 
day, was called an hour of that day; and as one-tweifth 
part of the time' that palled from fun-fetting to fun-rile, 
was called an hour of the night; not only did- the hours 
of day differ from the hours of night, but from one ano¬ 
ther, at all times, except'at the vernal and autumnal equi¬ 
noxes; hence it became neceffary, either to make the wa¬ 
ter fall irregularly into a receiving veil'd, with equidiftant 
hour-marks, or to have varying hour-marks for a regular 
efflux; the firft of thefe methods (which probably pre¬ 
ceded that of Ctefibius) was thus effefted, viz. A co¬ 
nical hollow veffel was inverted, or placed like a funnel 
in a frame, there being a very fmall aperture at the apex 
of the cone, and another folia cone, every way limilar as 
to dimenfions, was plunged into the hollow one, when 
filled with water to a greater or a fmaller depth, accord¬ 
ingly as the efflux was wanted to be more or Iefs rapid; 
and then adjufting-marks, correfponding to every day and 
night in the year, were put on a long Item, inlerted into 
the broad end of the folid cone, and kept in its polition 
by the frame, to Ihow how much the inner cone was to 
be depreffed or elevated, to accelerate or retard the ifi'ue 
of the fluid for the correfponding time ; there was a l'pout 
which fupplied a cOnftant influx of water, and a wafte- 
pipe, connefted with the top of the conical veffel, which 
carried oft' the fuperfluous water; hence the conftant in¬ 
flux of water prel'erved an unvarying height of the furface 
from the aperture, which.aperture was varied at plealure, 
by t’he elevation or depreffion of the inner cone. If now 
we fuppofe the l'ubjacent veffel to be a cube, cylinder, or 
any other regular figure, and equidiftant hour-marks to 
be properly made on its fide, the furface of the water, or 
an index borne by it on a piece of cork, would, as it rpfe, 
indicate the hours correfponding to thofe marks. The 
imperfections of this clepfydra were thefe: it required 
two daily manual adjuftments, one in the mornijig, and 
another in the evening ; and it made no allowance for the 
variation of fluidity, in different degrees of temperature, 
which, it is afi'erted, (hut perhaps without proof,) greatly 
influenced the ifochronifm of the drops. As an improve¬ 
ment, or rather appendage, to this conftruftion of the 
clepfydra, a bar, with rack-work at the upper endj was 
made, to float on the furface of the lower velfel by means 
.of 
