The Clepsydra.
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.
In taking photographs of the sun with the full aperture of this telescope, no driving mechanism is necessary. On the contrary, the difficulty is rather to arrange the apparatus so that an exposure short enough may be given to the sensitive plate, and solarization of the picture avoided. It is not desirable to reduce the aperture, for then the separating power is lessened. The time required to obtain a negative is a very small fraction of a second, for the wavy appearance produced by atmospheric disturbance is not unfrequently observed sharply defined in the photograph, though these aerial motions are so rapid that they can scarcely be counted. Some kind of shutter that can admit and cut off the solar image with great quickness is therefore necessary.
Fig. 36.