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.