s I L ^' E i; I.; I) a t. a s s t k l i: s c o p i^ . 



37 



Fig. 34. 



as pach drop of water escaped from the fuiniel-sliaped vessel, only a c-orrespondiiifr 

 weio-lit would be brou-ht into play. This is not the ease- with" sand, for in this 

 nistance every grain that passes out causes tjie whole weight that is sui)porte(l 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 slide being overcome by the steady application of the whole 

 mass of the weight. 



When these advantages in the flow of sand we>re ascertained, all tln^ other ])rime 

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

 dox, air-clocks, &c., in great variety, had be(m constructed. 



The sand-clock consisted of a tul)e a (Fig. ;U), eighteen inches long and one and 

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

 and sifted. Upon the top of the sand a leaden 

 weight b was placed. At the bottom of the 

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

 regulated the flow, the amount passing depend- 

 ing on tlie size of the aperture <L This stop 

 cock consisted of two thin plates, fixed at one 

 end and free at the other. The one marked c 

 is the adjusting lever, and its aperture mo^es 

 past tliat in the plate //. The lever/ serves to 

 turn the sand off altogether, Avithont disturbing 

 the size of the other aperture, which, once set 

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

 times. A movable co\"er Ii, perforated to allow 

 the cord / to pass throiigh, closed the top, wliile PijM~[il 

 the v(>ssel /,■ retained the escaped sand, which at \ *|i 

 suitable times was returned into the tube ;(, the 

 weight h being temporarily lifted out. From 

 the clock the cord / comramiicated motion to 

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

 possible, consistent with pliability, and well Avaxed. 



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

 of sand in such a hmg tube would be much quicker when the tube was full tlcni 

 wlu'U nearly empty, and that certainly that result woiUd occur when a heavv A\-eight 

 was put on the shifting mass. But in neither case have 1 been able to detect the 

 slightest variation, for, althougii 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. 'I'iu-se peculiarities seem to result from the sand urcliiug 

 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 (h\ sand. 



The Sand-Clock. 



