10 Mr. C. Packe on the Scales of the 



To reduce the barometer to the freezing-point, we have the 

 following form til se : — 



Let M = the cubic expansion of mercury for the number of 

 degrees Centigrade or Fahrenheit, by which the 

 observed temperature differs from the freezing-point. 

 B = the linear expansion of the brass scale for the num- 

 ber of degrees by which the observed temperature 

 differs from the standard. 

 H = the observed height of the barometer in inches or 

 millimetres. 

 The formula for the reduction of the English barometer will be 



Above 62° F -(M-B)xH 



Below 62°, above 32° . , - (M + B) x H 

 Below 32° +(M-B)xH 



For the reduction of the French barometer, 



Above the freezing-point . — (M— B) x H 

 Below the freezing-point . +(M-B)xH 



By the difference in the standard temperatures we can thus 

 account for '00936 inch out of the '017 by which the Kew 

 equivalent to the boiling-point differs from that of Regnault. 

 The remainder is almost exactly accounted for by the difference 

 of gravitation. 



The increase of gravitation, i. e. gravity as diminished by the 

 centrifugal force, is from the equator to the poles =' '0052005, 

 or -00260 to the 45th degree of latitude. 



Adopting the law that it increases as the square of the sine of 

 the latitude, we find that in 51° 30', the latitude of London, the 

 increase of gravity is ='0031852; in latitude 49°, that of Paris, 

 the increase of gravity '0029621 . 



Let D be the difference of these two gravities = '0002231 ; 

 B' the height of the mercurial column at London, equiva- 

 lent to a given pressure ; 

 B the corresponding height of the mercurial column at 

 Paris. 

 B will be equal (B'-f B'D) : e. g. 



Gravity in lat. 51° 3(y=-0031852 

 Gravity in lat. 49° = -0029621 



Difference = '000223 1 m log 4*348305 = D 

 29-905 = log 1-475 744=B' 



3-824049 = -006688 = B'I 



To compare, therefore, the barometric column at London 

 with that representing a corresponding pressure at Paris, we 



