Chap. II.
EGYPTIAN.
35
Ptolemies, the first machines for the purpose of raising water seem to have been in-vented. After Hippocrates had constructed tables which showed the exact motion of thesun , Clepsydra ?, or water-dials, were brought to great perfection by Scipio Nasica , thecousin of Scipio Africanus , who about two hundred years before Christ introduced theminto Home.
Ctesibius , who flourished in the reign of Ptolemy Physcus, an hundred a*nd twenty yearsbefore Christ, brought these machines into very general use, and invented the hydraulicorgan, which was operated upon by air and water. In the clepsydra he introduced,probably for the first time, toothed wheels: this instrument for the measurement of thehours was a cylinder resting upon a pedestal; two figures were placed upon the latter,one of which dropped water from its eyes, whilst the other pointed with a wand tothe hour marked on a vertical line drawn upon the cylinder. This cylinder turned onits axis once a year, and on it were drawn curved lines, which exhibited the inequalityof the hours on different days, by their being marked at unequal distances.
The manner of working this machine was to allow the water to rise through a tube,which, passing through one figure, was discharged by the eyes, into a reservoir, I\I, fromwhich it passes by a hole near m, into the pipe B, C, 1). In this pipe a piece of wood
ELEVATION.
SECTION.
floated upon the surface, and by its ascent, as the pipe filled, it raised the small pillar C, D,on which the other figure rested, and as the float rose in the pipe the wand was madeto point to the different hours. Every twenty-four hours the vessel became filled, asdid the inverted siphon, which communicated with it. The water was then drawn off'by the siphon, and falling in its descent into the buckets of the wheel below, put that intomotion. This wheel had six buckets, and therefore made one revolution every six days.Its axis carried a pinion of six teeth, working on another wheel of sixty teeth : thisalso carried another pinion of ten teeth, and drove a wheel of sixty-one teeth, which byits axis turned the pillar once round in 366 days.
These machines indicate considerable hydrodynamical knowledge, and derive theirorigin from the previous discoveries of Archimedes .
Another clepsydra received the water in a reservoir, which was always kept full, anddescended by a pipe into a hole formed in the great drum. This hole corresponded toone of the openings in the groove round the circumference of the small drum. The aper-tures of the groove in the small drum were of different sizes, to admit different quantitiesof water, according to the length of the day; and the proper aperture for the given daywas found by placing the index opposite the sun’s place on the zodiac, shown at N, theindex O being used for the night hours. The water which descended through the open-ings in the small drum was conveyed by the pipe F, through the aperture at G into thereservoir H. As the water rose in the reservoir, the inverted vessel, suspended by a chain,which passed round the axis R, and balanced by the counterweight, ascended and movedthe hour hand, which pointed to the dial plate.
d 2