It consists of a cylinder _a_, in which a piston _b_ moves watertight. At the top of the piston rod is a leaden five-pound weight _c_, from which the cord _i_ goes to the sliding plateholder _g_. The lower end of the cylinder terminates in a stopcock _d_, the handle of which carries a strong index rod _e_, moving on a divided arc. At _f_ a tube with a stopcock is attached. Below, a vessel _h_ receives the waste fluid. In using the clepsydra the stopcock of _f_ is opened, and the piston being pulled upwards, the cylinder fills with water from _h_. The stopcock is then closed, and if _d_ also is shut, the weight will remain motionless. The string _i_ is next connected with the slide, and the telescope turned on the moon. As soon as the slide is adjusted in angular position (page 36) the stopcock _d_ is opened, until the weight _c_ moves downwards, at a rate that matches the moon’s apparent motion. In order to facilitate the rating of the clepsydra, the index rod _e_ is pressed by a spring _k_ (2), against an excentric _l_. As the excentric is turned round, the stopcock _d_ is of course opened, with great precision and delicacy. The plug of this stopcock (3) is not perforated by a round hole, but has a slit. This causes equal movements in the rod _e_, to produce equal changes in the flow. The rating requires consequently only a few moments. The object of the side tube _f_ is to avoid disturbing _d_ when it becomes necessary to refill the cylinder, for when it is once opened to the right degree, it hardly requires to be touched again during a night’s work. In order to arrest the downward motion of the piston at any point, a clamp screws on the piston rod, and can be brought into contact with the cylinder head, as in the figure. That this instrument should operate in the best manner, it is essential to have the interior of the brass cylinder polished from end to end, and of uniform diameter. If any irregularity should be perceived in the rate of going, it can be cured completely by taking out the piston, impregnating its leather stuffing with fine rotten stone and oil, and then rubbing it up and down for five minutes in the cylinder, so as to restore the polish. The piston and cylinder must of course be wiped, and regreased with a mixture of beeswax and olive oil (equal parts) after such an operation. In replacing the piston, the cylinder must be first filled with water, to avoid the presence of air, which would act as a spring. Although it may be objected that this contrivance seems to be very troublesome to use, yet that is not the case in practice. Even if it were, it so far surpasses any prime mover that I have seen, where the utmost accuracy is needed, that it would be well worth employing. c. _The Sun Camera._