1 1 2 Chemical and Physical Notes 



distance of the sea-level in any locality from the centre of the 

 earth is at once ascertained by determining the force of gravity 

 there ; hence the gravitational method of determining the 

 figure of the earth. Here it must be remarked that a perfect 

 survey of the figure of the earth may be given without 

 affording any information about its size. 



To return to the barometric data in the table, we see that 

 while the height of our standard barometer was the same, 

 735'5 mm. at every station, the indications of the hypsometer 

 or the boiling-point varied. It has been shown that the height 

 of the barometer is not affected by change in the force of gravity, 

 but that the pressure of the atmosphere is so affected; we might 

 conclude that in the temperature of saturated steam, or the 

 boiling-point of water, at the sea-level, we have a method of 

 determining the force of gravity at any place at that level. 

 But we must be on our guard. Our knowledge of the relation 

 existing between the boiling-point of water and the pressure 

 on its surface is derived from observations of the barometer. 

 Therefore, from the boiling-point of water we cannot get 

 absolute data with regard to the force of gravity. But it may 

 be that it can afford information about the variations of the 

 force of gravity. 



For instance, we observe our standard barometer having 

 the temperature of melting ice at the sea-level, in lat. 20, 45 

 and 70, and at each of these places its height is 735*5 mm. ; 

 we apply the usual correction to reduce this barometric height 

 to the equivalent height when the mercury is exposed to the 

 same gravitational force of attraction. By convention, the 

 standard force of terrestrial attraction is taken to be that 

 exerted at the sea-level in lat. 45. The barometric heights 

 thus adjusted are now 734, 735-5 and 737 mm., and these 

 columns of mercury under the influence of standard gravity 

 exercise pressures of 1002*04, roooand 997-8 grms. per square 

 centimetre. It results from experiments, principally by 

 Regnault, on the relation between the boiling-point of water 

 and the height of the standard barometer, that at these 

 true pressures water boils at 99'O29, 99-o87 and 99'i42 C. 

 respectively. 



