We are well assured that the ancients had machines by which, through
the help of water, they were able to measure time[197]. The invention
of them is by Vitruvius[198] ascribed to Ctesibius of Alexandria,
who lived under Ptolemy Euergetes, or about the year 245 before the
Christian æra[199]. They were introduced at Rome by P. Cornelius
Scipio Nasica, in the year 594 after the building of the city, or
about 157 years before the birth of Christ. How these water-clocks
were constructed, or whether they were different from the clepsydras,
I shall not inquire. If under the latter name we understand those
measurers of time which were used in courts of justice, the clepsydra
is a Grecian invention, first adopted at Rome under the third
consulship of Pompey[200]. The most common kinds of these water-clocks
all, however, corresponded in this, that the water issued drop by drop
through a hole of the vessel, and fell into another, in which a light
body that floated marked the height of the water as it rose, and, by
these means, the time that had elapsed. They all had this failing in
common, that the water at first flowed out rapidly, and afterwards more
slowly, so that they required much care and regulation[201].
That ingenious machine, which we have at present under the name of
a water-clock, was invented in the seventeenth century. The precise
time seems to be uncertain; but had it been before the year 1643[202],
Kircher, who mentions all the machines of this kind then known, would
in all probability have taken notice of it. It consists of a cylinder
divided into several small cells, and suspended by a thread fixed to
its axis in a frame on which the hour distances, found by trial, are
marked out. As the water flows from the one cell into the other, it
changes very slowly the centre of gravity of the cylinder, and puts
it in motion[203]; much like the quicksilver puppets invented by the
Chinese[204].
These machines must have been very scarce in France in 1691; for
Graverol at that time gave a figure and description of the external
parts of one, but promised to give the internal construction as soon as
he should become acquainted with it[205]. This was the only one then in
Nismes. He says, also, that they were invented a little before by an
Italian Jesuit, who resided at Bologna, but were brought to perfection
by Taliaisson, professor of law at Toulouse, and a young clergyman
named De l’Isle.
Alexander says more than once that this machine was invented at Sens
in Burgundy, in 1690, by Dom Charles Vailly, a Benedictine of the
brotherhood of St. Maur, and that he brought it to perfection by the
assistance of a pewterer there, named Regnard. This account is in
some measure confirmed by Ozanam; for he says expressly, that the
first water-clocks were brought from Burgundy to Paris in 1693, and he
describes one which was made of tin at Sens. Dom Charles Vailly was
born at Paris in 1646, and died in 1726; he was celebrated on account
of his mathematical knowledge, though he is known by no works, as he
burned all his manuscripts[206].
Alexander, however, who was of the same order, seems to have ascribed
to his brother Benedictine an honour to which he was not entitled; for
Dominic Martinelli, an Italian of Spoletto, published at Venice, in
1663, a treatise written expressly on these water-clocks, which Ozanam
got translated into French by one of his friends, and caused to be
printed with his additions[207]. This translator says that water-clocks
were known in France twenty years earlier than Ozanam had imagined. It
appears therefore that they were invented in Italy about the middle of
the seventeenth century, and that Vailly, perhaps, may have first made
them known in France[208].
It may perhaps afford some pleasure to those who are fond of the
history of the arts, to know that Salmon, an ingenious pewterer at
Chartres in France, has given very full and ample directions how
to construct and use this machine[209]. He is of opinion that the
invention is scarcely a century old; and that these water-clocks, which
are now common, were first made for sale and brought into use among the
people in the country, by a pewterer at Sens in Burgundy. What this
artist affirms, that they can be constructed of no metal so easily, so
accurately, and to last so long as of tin, is perfectly true. I have
however in my possession one of brass, which is well constructed; but
it suffers a little from acids. Among the newest improvements to this
machine may be reckoned an alarum, which consists of a bell and small
wheels, like those of a clock that strikes the hours, screwed to the
top of the frame in which the cylinder is suspended. The axis of the
cylinder, at the hour when one is desirous of being wakened, pushes
down a small crank, which, by letting fall a weight, puts the alarum in
motion. A dial-plate with a handle is also placed sometimes over the
frame.
[A very ingenious application of the principle of the clepsydra, for
the purpose of measuring accurately very small intervals of time, is
due to the late Captain H. Kater. Mercury is allowed to flow from a
small orifice in the bottom of a vessel, kept constantly filled to
a certain height. At the moment of noting any event, the stream is
interrupted and turned aside into a receiver, into which it continues
to run till the moment of noting any other event, when the intercepting
cause is suddenly removed. The stream then flows in its original
course. The weight of mercury in the receiver, compared with the weight
of that which passes through the orifice in a given time, observed by
the clock, gives the interval between the events.]