272 0N THE VELOCITY OP AtR 



of 6 feet is to the velocity produced by that head, fo is the 

 fquare root of any other depth to the velocity produced by that 

 depth, 

 or In bellows, We may alfo, from the above, conftruct an inftrument which 

 will (hew the velocity with which air flows out of any kind of 

 bellows, with as much accuracy, as the experiments have been 

 made on which its conftruclion depends. 



Defcription of the Inftrument, &c. 



by means of an The metal box or tube B, plate xiv. fig. 2, may be about 



inftrument here the fize of the figure; the top muft be made air tight by the 



a^ortable veflcl cover L) into the bottom is fixed the fmall tube A C, and into 



containing wa- the piece D is cemented the glafs tube ED; the inftrument is 



ter, to be form- th i nverte d, and fome water poured through the tube A C, 



ed into an open ; ^ . K . & . 



gage tube > till, when in its proper pofition, it is vifible at D. It is now 



ready for ufe, and the end A may be fixed in a hole made in 



the upper board of the bellows, and the water will rife in the 



glafs tube, in fmith's bellows, generally from 9 to 12 inches, 



furnace bellows, generally four feet or more. But where the 



compreflion is great, quickfilver may be ufed inftead of water, 



only in this cafe the inftrument fhould be made of iron, as 



quickfilver caufes the fcrews of brafs to fcreak. Or, inftead of 



quickfilver, the tube E D may be fealed at the top E, and then 



a length of 12 or 18 inches will be enough for any blaft. The 



glafs tube needs not be more than one-eighth or one-tenth of 



an inch in diameter. 



Whatever compreffion there may be in the bellows, there 



will be the fame in the upper part of the tube B, which will 



force the water into the glafs tube DE, and make the air in its 



upper part of the fame denfity, deducting from the compreffing 



force the altitude of the water raifed above D, which however 



will be of little or no importance; if the gauge is placed in a 



horizontal pofition, with the glafs tube downward, there will 



be no difference of denfity. 



the tub- may The computation for the force in the cafe where a tube her- 



be hermetically metically fealed at the top is adopted in the inftrument, will be 



cIofed * effected by confidering that the fpace occupied by any elaftic 



fluid is inverfely as its force. Thus, let the tube be 12 inches 



long, and fuppofe the water to be raifed one inch, then it will 



be 11 : 12 :: the force of the atmofphere : the force of the air 



in the tube : ; 1 : 1 T f T . Hence a fcale may be adapted to the 



inftrument, 



