THE BATHOMETER. 129 



with another liquid of less density, with water or spirits of 

 wine, and this other liquid terminates in a spiral tube laid 

 upon a scale at the top of the instrument. The centre of 

 the diaphragm which supports the column of mercury is 

 carried by two springs, or by four as in the case before you, 

 which are made of carefully-tempered steel and are 

 stretched out to such a degree that the elastic pressure 

 exercised by these springs exactly balances the dead weight 

 of the column of mercury resting upon this diaphragm, the 

 result being that the diaphragm retains its horizontal 

 position. This column of mercury presses with a force 

 resulting from the area of the diaphragm multiplied by the 

 height of the column, and it will be seen at once that this 

 force is considerable. This diaphragm is 90 millimetres in 

 diameter, and the height of the column is 600 millimetres. 

 The total force of gravitation resisted by this diaphragm is 

 equal to about 120 pounds, and therefore any variation in 

 the total gravitation would be a variation in the weight of 

 over a hundredweight, and the g-.-guiy.ow P ar ^ ^ this amount 

 still a measurable quantity. The central part of this 

 column has been reduced. Instead of having the column 

 of mercury from end to end in a parallel cylinder, it is 

 reduced in the middle to a much smaller area, and this is 

 not done without an object. If the mercury stood in a 

 column of equal diameter throughout, and if the area of 

 this column remained the same under all circumstances of 

 temperature, it would be easily seen that the potential or the 

 total pressure of this column of mercury upon the diaphragm 

 would be always the same, whatever the temperature 

 or the state of expansion by temperature of the mercury 

 might be, because in proportion as the density of mercury 

 is diminished through dilatation, the height of the column 

 would increase and the one would exactly compensate the 

 other. But we have to consider that the balancing force 

 to the force of gravity is a variable force, the elastic force 

 }f the spring ; and I had to determine by careful experiment 

 what is the amount of variation which this elastic force 

 undergoes for a given variation of temperature. Wertheim 

 in Germany had already investigated this subject, but 

 hardly to the extent necessary to guide me in the construc- 

 tion of this instrument. The result of my experiments, 

 which on the whole agree with those of Wertheim, proves 



VOL. II. K 



