VOL. XHV.] PHILOSOPHICAL TRANSACTIONS. 237 



in g2, marked s t i, of which s the mouth, is 18 inches above the bottom of 

 the vessel g, and 18 inches below the top of the tantalus t; i is the issuing leg 

 of the tantalus, which discharges the water out of the vessel g into the cup z, 

 as soon as it runs over the top t, till the water sinks as low as s. 



Fig. 9 shows the profile of the clepsydra. And fig. 10 the plan of the same. 



The case uu incloses the whole machine, except the cistern that supplies the 

 siphon d, which may be placed at any distance from it, as is most convenient, 

 provided the issuing leg d, of the siphon is lengthened out so as to give a con- 

 stant stream into the canal e. This case uu supports the axis of the cylinder o 

 behind, and the dial plate pp before; in the centre of which turns the axis o, 

 with the index k at its extremity, being the minute hand. The hours may be 

 described by two common wheels, as in ordinary clock-work. For cheap work, 

 chains passing round pulleys would do, instead of wheels with teeth. 



The motion of the clepsydra is effected in the following manner: the short leg 

 of the siphon d is placed in a cistern, with its mouth something below the mouth 

 of the waste-pipe ; which cistern is supplied with a constant stream, rather more 

 than what runs out at the siphon d, which overplus going off at the waste-pipe, 

 the water always remains at the same height in the cistern, and yet always delivers 

 a constant and equal flow into the canal ee ; consequently there is not the least 

 intermission. As the end of the canal e, fixed to the pipefl, is in the figure the 

 lowest, the water runs all through the pipefl, into the vessel gl, till it runs 

 over the top of the tantalus t ; when it immediately runs out at i into the cup z, 

 at the end of the balance m, and forces it down, the balance m moving on its 

 centre v. When one side of m is brought down, the string which connects it to 

 fl, running over the pulley 1, raises the end fl, of the canal e, which turns 

 on its centre, higher than f2 ; consequently all the water which constantly 

 runs through the siphon d, instantly runs through f2 into g2, till the same ope- 

 ration is performed in that vessel, and so alternately. 



As the height the water rises in g in an hour, viz. from s to t, is equal to the 

 circumference of n, the float y rising that height along with the water, lets the 

 weight x act on the pulley n, which carries with it the cylinder o ; and, giving a 

 revolution, makes the index k describe an hour on the dial-plate. This revolu- 

 tion is performed by the pulley nl ; the next is to be by n2, while nl goes back, 

 as the water in gl runs out through the tantalus ; for y must follow the water, 

 as its weight increases out of water. 



The axis o always keeps moving the same way ; the index k describes the mi- 

 nutes ; the tantaluses must be wider than the siphon d, that the vessels gg may 

 be sure to be em ty as low as s, before the water returns to them. 



