38 ON T H E C O N S T II U C T I O N AND US E O E 



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

 where steady motion and a laro;e amount of power are demanded. The simplicity, 

 for instance, of a heliostat on this plan, the large size it might assvune, and its 

 small cost, would be great recommendations. In tlit^se respects its advantages over 

 wheelwork are very apparent. The precision with A\liich 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 manj' times the period re([nir(Hl to ])hotograpli it. To secure 

 the greatest accuracy in tlu; rate of a sand-clock, some precautions must be taken. 

 The tube 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 

 hygrometric 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. 'J'lu; weight, which may b{> of lead, must be turned so 

 as to go easily down tlie tube, and must be co\cred witli writing paper or some 

 other hard and smooth materials to a\()id tlie jironeness to adhesion of sand. A 

 long bottle filled witli mercury answers well as a substitute. 



I have used in such clocks certain metallic ])reparations : Fine shot, on account 

 of its equality of size, might do for a very large clock with a considerable opening 

 below, but is unsuitable for a tulx' of the size stated above. Tliere is, however, 

 a method by which lead can l)e reduced to a divided condition, like fine gunpowder, 

 when it may re])lace tlu' sand. If that metal is melted with a little antimony, and 

 while cooling is shaken in a box containing some ])lnmbago, it breaks up at the 

 instant of solidifying into a fine ])owder, which is altout five times as heavy as sand. 

 If after being sifted to select tlie grains of ])roi)er size, it is allowed to run through 

 a small hole, the flow is seen to be entirely different from that of sand, looking as 

 if a Avire or solid rod were descending, and not an aggregation of particles. It is 

 probable, therefore, that it would chj better than sand for this purpose. I have not, 

 however, given it a fair trial, because just at the time when the experiments with the 

 sand-clock had reached this point, I determined to try a clei)sydra as a prime mover. 



The reason which led to this change was that it was observed on a certain occa- 

 sion when the atmosphere was steady, that the ])hotogra])hs did not correspond in 

 sharpness, being hi fact no better than on other nigiits when there Avas a consider- 

 able flickering motion in the air. A fintlier investigation showed that in these 

 columns of sand there is apt to he a minute \ibrating movement. At the ])late- 

 holder above this is converted into a series of arrests and advances. On some 

 occasions, however, these slight deviations from continuous motion are entirely 

 absent, and generally, indeed, they cannot be seen, if the parts of the image seem 

 to vibrate on account of currents in the air. By the aid of the microscopic exag- 

 geration described on a former page — which was subse([uently put in practice — 

 they may be observed easily, if present. 



When the negative produced at the focus of the great mirror is intended to be 

 enlarged to two feet or more in size, these movements injure it sensibly. A variety 

 of expedients was resorted to in order to avoid them, but none ])roved on all 

 occasions successful. 



It is obvious that in a water-clock, where the mobility of the Hiiid is so Tnuch 



