Chemical and Physical Notes 109 



The gravitation correction for the barometer, as supplied 

 in meteorological instructions, is based on the assumption of 

 a homogeneous earth, and refers only to varying distance of 

 the station from the centre of the earth. The force of gravity 

 varies inversely with the square of this distance. It is obvious 

 that there will be some difference between the force of gravity 

 at the level of the sea and that at the top of a high mountain 

 in the neighbourhood. We have taken the mean radius of the 

 sphere to be 6371 kilometres. At a height of 1000 metres 

 above the sea the force of gravity is less than it is at the level 

 of the sea in the proportions 63722 : 637 1 2 = 0-99968. At a 

 height of 5000 metres the relative reduction is 0*99843, and 

 at 10,000 metres it is 0-99687. 



Owing to the spheroidal shape of the earth, places at the 

 sea-level are at different distances from the centre of the earth 

 if they are situated in different latitudes. 



The most recent values of the dimensions of the earth's 

 spheroid are : 



Semi-axis major ... ... ... 6378 kilometres 



Semi-axis minor ... 6356 



Difference 22 



The greater radius is the equatorial and the smaller the 

 polar, therefore the force of gravity at sea-level is a maximum 

 at the poles and a minimum at the equator, and the relation 

 between them is 0-99311. The difference between the equa- 

 torial and the polar radii is about three times the height of 

 the highest mountain on the earth's surface. 



In order to illustrate these remarks Table XVIII has been 

 constructed. It is supposed that the barometer has been 

 observed at a temperature of o C., and at the sea-level in the 

 latitudes given in the first column, and that it stands always 

 at a height of 735'5 millimetres. As the force of gravity at 

 the sea-level increases from the equator to the pole, this 

 constant barometric height corresponds to a true atmospheric 

 pressure, which increases as the pole is approached. This is 

 given in the second column in grammes per square centimetre. 

 In the third column these pressures are given in terms of the 



