134 Celestial Photography. 



complete. Excellent photographs were taken by means of a 

 slide thus supported, and driven by the " sand-clock/' which 

 consisted of a weight supported by a column of sand allowed 

 to escape through an orifice whose dimensions were variable at 

 pleasure to suit the required rate : and this arrangement was 

 found superior to mercury-clocks and air-clocks, which had been 

 tried in great variety. In fact, Dr. Draper expresses his belief that 

 no prime mover is more suitable than a sand-clock for purposes 

 where steady motion and a large amount of power are demanded. 

 te The precision," he says, ' ' with which such a sand-clock 

 goes, may be appreciated when it is stated that under a power 

 of 300 a lunar crater can be kept bisected for many times the 

 period required to photograph it." Some precautions are how- 

 ever requisite, as might be expected, to secure accuracy of rate. 

 The tube containing the sand should be " free from dents, of 

 uniform diameter, and very smooth or polished inside. Water 

 must not be permitted to find access to the sand, and hygro- 

 metric varieties of that substance should be avoided, or their 

 salts washed out. The sand should be burned to destroy 

 organic matter, and so sifted as to retain grains nearly equal 

 in size. The weight, which may be of lead, must be turned so 

 as to go easily down the tube, and must be covered with writ- 

 ing-paper or some other hard and smooth material, to avoid the 

 proneness to adhesion of sand." He found, however, that these 

 columns of sand were frequently liable to minute vibrations, 

 which caused want of sharpness in the enlarged photograph, 

 and an ingenious experiment was tried to obviate these, by 

 substituting metallic for natural sand. The former may be 

 prepared by melting lead with a little antimony, and shaking 

 it while cooling in a box containing some plumbago, the effect 

 of this being to solidify it in the form of a fine powder, about 

 five times as heavy as sand, which only requires sifting to 

 secure uniformity of size : when allowed to escape through a 

 small tube, its flow is '* entirely different from that of sand, 

 looking as if a wire or solid rod were descending, and not an 

 aggregation of particles." It would probably have answered 

 better than sand, but the experiment was pushed no further, 

 as a trial of a water-clock or clepsydra proved perfectly suc- 

 cessful. This consists of a vertical cylinder with a water-tight 

 piston, which carries at its top a 51b. weight to move the 

 sliding plate-holder ; the lower end of the cylinder being ter- 

 minated by a stop-cock, to regulate the escape of the water. 

 There is also another stop-cock in a pipe proceeding from the 

 lower part of the cylinder, through which, independently of 

 the first stop-cock, when the piston is raised, the cylinder fills 

 itself with water from a vessel underneath. This might of 

 course be accomplished by a single stop-cock, but the object 



