561 



be made at little expense, and sent to Sierra Leone and the 

 Pontine Marshes; in the latter place the pumps might be 

 worked by water conveyed from the mountains or other 

 cheap motive power. 



Dr. Apjohn read a paper " On the hygrometric Cor- 

 rection in barometric Formulae for the Measurements of 

 Heights. 



If the atmosphere were of one uniform temperature 

 throughout, destitute of moisture, or in a constant hygrome- 

 tric condition, and if the intensity of gravity were also con- 

 stant, it is well known that the difference of the altitude of 

 any two points in the atmosphere would be represented cor- 



n 

 rectly by the formula D = /w X log. -^, m being a constant 



quantity, and p and p' being the respective pressures at the 

 lower and upper stations, as measured by the barometer, or 

 in any other way. A correction for temperature has been 

 long applied by augmenting or diminishing the approximate 



heisht, or »J X log. -^, by the amount that a column of air 



of this length would expand or contract if its temperature 



t A- B 

 were changed from 32° to — ^ — , t being the temperature of 



the lower, and B that of the upper extremity of the jerial 



column, by which the expression becomes 



/ 1+»-32n 

 Dzz«.xlog.^x(^l + -^^^— j. 



Such is, I believe, a correct account of the present form 

 of the barometric formula, at least when we neglect the cor- 

 rection for variations of gravity, which is, however, in gene- 

 ral so small as to be safely negligible. The presence of 

 moisture in the air, or rather its varying amount, must ob- 

 viously exercise some disturbing effect on this formula ; but 

 3a2 



