262 A. G. Howard. — The Barometer : [Sept. 28, 



inches, by reducing up Cape Town will read 25*94 inches, and 

 Kimberley 25*98 inches, thus reversing the gradient. 



In reducing down to the sea level the conditions are totally different. 

 It is true that this is an attempt to delineate an impossibility, but of 

 "^the two impossibilities it is the least objectionable. 



If we assume the whole of the solid earth above the mean sea level 

 to be so much atmosphere, perfectly tranquil and unaffected by 

 temperature, currents or any movement, and in layers of diminishing 

 density, the atmosphere itself rolling over it as it does over the solid 

 earth. If we again assume that the weight of the atmosphere above 

 this mass affects it by compression, so that the surface pressure shall 

 bear a proportional ratio to that at the sea level, we have data for 

 constructing reduction tables, for varying heights, provided we know 

 the decreasing ratio of atmospheric density. 



It has been demonstrated that the density of the atmosphere 

 decreases with the squares of the distances, and that the number 

 representing this decrease of density obtained by dividing the pressure 

 at any altitude into 30 inches, increases in the proportion of the power 

 of the quotient of altitudes, or in other words, if B represents the 



upper pressure then log. — - increases directly as the altitude, so 



30 

 that by applying a correction to the height, 'the log. of — - will be 



obtained. 



Hence we have the resolvable equation — 



H = log. ^ kg (1) 



Where H is the altitude in feet, and B the pressure at that altitude, 

 ^ is a constant and g the latitude correction. 



30 

 To find A, let a represent log. — -. Then, omitting g we have, 



B 



ah, =r H, and consequently — 



A = 5 (2) 



a 

 So that by taking the mean of a number of readings at two 

 stations, the one at sea level and the other at a known altitude, we 

 can easily work out h, I have calculated this from readings at 

 ^Graham's Town and Port Alfred, and find it to be 65,000. This 

 number varies slightly with the sea-level pressure, varying from 

 64,900 to 65,100, from a pressure of 29 to 30 inches. It is also the 

 same for all altitudes. 



