A SILVERED GLASS TELESCOPE. 



37 



as each drop of water escaped from the funnel-shaped vessel, only a corresponding 

 weight would be brought into play. This is not the case with sand, for in this 

 instance every grain that passes out causes the whole weight that is supported by 

 the column to come into action. In the former instance a movement consisting of 

 a series of periods of rest and periods of motion occurs, because power has to accu- 

 mulate by floating weight lagging behind the descending water, and then suddenly 

 overtaking it. In the latter case, on the contrary, there is a regular descent, all 

 minor resistances in the sHde being overcome by the steady appUcation of the whole 

 mass of the weight. 



When these advantages in the flow of sand were ascertained, aU the other prime 

 movers were abandoned. Mercury-clocks, on the principle of the hydrostatic para- 

 dox, air-clocks, &c., in great variety, had been constructed. 



The sand-clock consisted of a tube a (Fig. 34), eighteen inches long and one and 

 a half in diameter, nearly filled with sand that had been raised to a bright red heat 

 and sifted. Upon the top of the sand a leaden 

 weight 6 was placed. At the bottom of the P"g- 34. 



tube a peculiar stopcock, seen at (2) enlarged, 

 regulated the flow, the amount passing depend- 

 ing on the size of the aperture d. This stop- 

 cock consisted of two thin plates, fixed at one 

 end and free at the other. The one marked e 

 is the adjusting lever, and its aperture moves 

 past that in the plate g. The lever / serves to 

 turn the sand off' altogether, without disturbing 

 the size of the other aperture, which, once set 

 to the moon's rate, varies but slightly in short 

 times. A movable cover 7j, perforated to aUow 

 the cord i to pass through, closed the top, while 

 the vessel li retained the escaped sand, which at 

 suitable times was returned into the tube a, the 



weight & being temporarily hfted out. From ,^^ Sand-ciock. 



the clock the cord % communicated motion to 



the frictionless slide, as shown in Fig. 33. This cord should be as inelastic as 

 possible, consistent with pliability, and well waxed. 



One who has not investigated the matter would naturally suppose that the floAV 

 of sand in such a long tube would be much quicker when the tube was full than 

 when nearly empty, and that certainly that result would occur when a heavy weight 

 was put on the shifting mass. But in neither case have I been able to detect the 

 slightest variation, for, although by shaking the tube a diminution of the space 

 occupied by the sand may be caused, yet no increase of weight tried could accom- 

 plish the same reduction. These peculiarities seem to result from the sand arching 

 as it were across the vessel, like shot in a narrow tube, and only yielding when the 

 under supports are removed. In blasting, a heavy charge of gunpowder can be 

 retained at the bottom of a hole, and made to split large masses of rock, by filling 

 the rest of the hole with dry sand. 



