APPENDIX A. 233 



Placing the instrument under the receiver and 

 forming a partial vacuum, the index will rise into 

 the enlargement. Then, admitting the air by de- 

 grees and very slowly, we may note the correspond- 

 ence between the heights of the ordinary mercury 

 manometer and the point which will be reached 

 by the lower face of the index of the instrument. 

 This will answer to form a comparative table of 

 the pressures and the numbers of the scale. The 

 pressures would be represented by their relations 

 to the observed pressure at the moment of the 

 passage of the index over zero, for any other fixed 

 number of the scale. 



Thus, for example, suppose that we observed on 

 the manometer 400 or n millimetres of mercury 

 when the index is on o, then n' when the index is 

 on 1, n" when on 2, and so on. This will give the 



n' n" 



ratios ,,... which must be inscribed in the 

 n n 



table. Then n could be varied at pleasure, and 

 the table could still be used. 



In fact, according to the law of Mariotte, vol- 

 umes preserving the same ratios, pressures should 

 also preserve the same ratios to each other. 



Let p be the pressure when the index is on o, v 

 the volume of air at the same moment, p' and v f 

 the same pressures and volume at the moment 



