MISCELLANEOUS PAPERS. 



352 



end of the beam on which the phial is fufpended, a weight 

 IS hung fufficient, with the weight of the phial, to over- 

 balance the regifter, and raife It, and confequently open 

 the flue. When the flue is opened to a due degree, the 

 regifter is held in that fituation, until fo much water is 

 poured into the copper through the cock, as will fill one 

 third of the veffel; then fhut the cock, and pour water 

 into the cylinder, until it rifes high enough to float the 

 phial. By pouring water into the cylinder, the air in the 

 veffel is compreflxd, and finding no way to efcape, as the 

 vefl^el is airtight, it refifls the water, and prevents its oc- 

 cupying the whole fpace; and therefore the upper part of 

 the veffel is apparently empty. The phial is loaded with 

 fhot, fo that it will fwim about one third above the water. 



When the water rifes in the tube, the phial riles w^ith It, 

 in which cafe the regifter A is fo ballanced, that it de- 



fceuds, and clofcs the flue. * 



After this defcription, the principles on which the Sen- 

 tinel Regifter a£ts, muft be obvious to every perfon ac^ 

 quainted with the elafticity of the air, and that this elaf- 

 ti(?ity is encreafed by heat. For when the fire In the fur- 

 nace is encreafed, the degree of heat in the flue is alfo en- 

 creafed; this encreafes the elafticity of the air contained 

 between the double bottom and fides of the copper, and 

 confequently of that, which occupies the fpace above the 



water, as there is a communication by means of the holes 

 already defcribed. The elafticity of the air being encreaf- 

 ed it expands, and by its expanfion forces the water up 

 the tube; the water being raifed, carries the phial with it, 

 whereupon the regifter preponderating defcends, clofcs tl 

 flue, and by leffening the draught of the chimney or flue, 

 deadens or checks the fire in the furnace. By this means 



ain the heat in the flue is diminiflied, the air in the ca*- 

 vhy becomes cooler, and confequently lefs elaftic, where- 

 upon the water defcends in the tube, and with it the phial 

 to its ftationary point. By the defcent of the phial the 

 -egifteris raifed, and opens the flue; by which oicaas it 



, ftands 



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