666 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 762. 



equally niised in the tube ; but the water is found by the experiments above re- 

 lated, to rise -xVo ^^ a" i"ch more than the mercury ; and therefore the water 

 must expand so much more than the mercury by removing the weight of the 

 atmosphere. 



In order to determine how much the water was compressed by this, or a greater 

 weight, he took a glass ball of about an inch and -^ in diameter, which was joined 

 to a cylindrical tube of 4 inches and -^v in length, and in diameter about , ^^ of 

 an inch : and by w^eighing the quantity of mercury that exactly filled the ball, and 

 also the quantity that filled the whole length of the tube ; he found that the 

 mercury in ^Vo- of an inch of the tube, was the 1 00000th part of that contained 

 in the ball ; and with the edge of a file he divided the tube accordingly. 



This done, he filled the ball and part of the tube with water exhausted of air ; 

 and left the tube open, that the ball, whether in rarefied or condensed air, might 

 always be equally pressed within and without, and therefore not altered in its di- 

 mensions. Now by placing this ball and tube under the receiver of an air-pump, 

 he could see the degree of expansion of the water, answering to any degree of 

 rarefaction of the air ; and by putting it into a glass receiver of a condensing en- 

 gine, he could see the degree of compression of water, answering to any degree of 

 condensation of the air. But great care must be taken in making these experi- 

 ments, that the heat of the glass ball be not altered, either by the coming on of 

 moisture, or its going off by evaporation ; which may easily be prevented by keep- 

 ing the ball under water, or by using oil only, in working the pump and con- 

 denser. 



In this manner he found by repeated trials, when the heat of the air was about 

 50 degrees, and the mercury at a mean height in the barometer, that the water 

 will expand and rise in the tube, by removing the weight of the atmosphere, 4 

 divisions and ^V, or one part in 21740 ; and will be as much compressed under 

 the weight of an additional atmosphere. Therefore the compression of water by 

 twice the weight of the atmosphere, is one part in lOSfO of its whole bulk.* 



The famous Florentine experiment which so many Philosophical writers have 

 mentioned as a proof of the incompressibility of water, will not, when carefully 



• If the compressibility of the water was owing to any air that it might still be supposed to con- 

 tain, it is evident that more air must make it more compressible; he therefore let into the ball a bub- 

 ble of air that measured near -^ of an inch in diameter, which the water absorbed in about 4 days ; 

 but he found on trial, that the water was not more compressed by twice the weight of the atmosphere^ 

 than before. The compression of the glass in this experiment, by the equal and contraiy forces 

 acting within and without the ball, is not sensible : for the compression of water in two balls, ap- 

 pears to be exactly the same, when the glass of one is more than twice the thickness of the glass of 

 the other. And the weight of an atmosphere, which he found would compress mercury in one of these 

 balls but J part of a division of the tube, compresses water in the same ball 4 divisions and -j^. — Orig. 



