CLEPSYDRA, or WATER Ctocx. A contrivance of very great antiquity, to measure the lapse of time, and Indicate the hour by the flowing of water into or out of a vessel properly graduated. In the former case the vessel was divided by lines into a number of equal arts, which would be filled with water in equal por tions of time provided the source from whence the water was obtained was so abundant as to render the hydrostatic pressure upon the discharging aperture nearly equal. Such clocks, how ever, could be employed in particular situations only, the latter plan was therefore most commonly adopted, viz. to measure by the discharge of water from the vessel; but as in this plan the velocity of the water would not be uniform, but would decrease with the decrease of the water in the vessel, equal quantities of water would not be discharged in equal times, and some contrivance became necessary to compensate this inequality. The most common was to employ a vessel oflarger diameter at the upper than at the lower end, and divide the altitude into a number of equal parts, or else to use a vessel of equal diameter throughout, and divide the height into a number of unequal parts, gradually diminishing from the to ; but each of these methods was difficult to execute accurately. The cut in the margin represents a contrivance of Mr. Par tington's, by which these difficulties are avoided, and equal quantities of water are discharged in equal spaces of time. a
is a cylindrical tube to hold the water, and b a cork float on its surface, through which is passed the shortest leg of a narrow syphon c, which is suspended by a silken cord over a wheel d, and to the other end of the cord is attached a weight, which nearly counterbalances the syphon ; near to the extremity of the syphon is fixed an index f, that points out upon a graduated scale the hour, according to the degree of depression within the tube. It is obvious that as the float by which the syphon is supported is always immersed to the same depth in the water, the outer leg of the syphon will always remain in the same relative position to the surface of the water in the tube, and thus the hydrostatic pressure will be always the same, the flow or discharge will be uniform, or equal portions will be discharged in equal times, and the tube being cylindrical, or of the same dimension throughout its length, the scale cor responding to its altitude will be divided into equal parts ; thus the instrument forms a very accurate measurer of time. The water falls into a receiver g, which forms the base of the instrument, and may be made to run back into the tube by opening a passage between them, and inclining the instrument.