SS Mr, Nixon on the Theory/ of the [Avd'J 



face of the earth at any latitude from the nearest point of the 

 axis. Thus it has been demonstrated that if we call the force 

 of gravity in latitude 45*^ equal to unity, then will its intensity 



at the equator be diminished -— -. : at the poles it will be aug- 

 mented in the same proportion. For any intermediate latitude, 

 the fraction must be multiplied by cosine oi' double the latitude. 

 In stating the number of measures of dry air required to coun^ 

 terpoiae one of mercury, we remarked that the barometer indi- 

 cating the pressure was understood to be stationed at the level 

 of the sea in latitude 45°. Supposing, on repeating the experi- 

 ment, that the force of gravity has diminished in the interim so 

 as to coincide with its value at the equator, let us inquire what 

 will be the consequences. In the first place, as the absolute 

 weights of the atmosphere and the mercury are diminished in 

 the same ratio, the height of the barometer will continue 

 unchanged. Secondly, as the absolute weight of each particle 



of air is diminished -rp without impairing its elastic force, the 



original pressure on any or all the strata is enfeebled in the same 

 proportion, and the height of the atmosphere, or of any portion 

 of it, is increased in the same degree. Thirdly, the particles 

 being more distant from each other, there is a consequent 

 increase of volume without addition of weight relative to that of 



the mercury, and a quantity of dry air greater by —z than had 



been required in the previous experiment, will be necessary to 

 balance the measure of mercury. Had we hermetically closed 

 the vessel containing the air in latitude 45°, and then transported 

 it to the equator, it would still be found to be an exact counter- 

 poise to a volume of mercury equal to that made use of in the 

 former latitude; but on opening the vessel and allowing the 

 included air to'communicate with the atmosphere (the barometer 

 there standing at the same height as when at the pole), then 

 would the ajr, being of an elasticity superior to the absolute 

 pre^sui*e^^TiOw irrciimbent on it, immediately expand, and a por- 

 tion of it 'rushing but, the residue would be insufficient to 

 balance the incompressible mercury. : m- 'i ' < 'ui 



The better to jHustrate the variation of/ih^Mdensilyvof air 

 resiiltin^'frohi^ change of latitude, place a proper weight 'yjn a 

 vertical colurim of elastic wire coded in the manner of a screw; 

 then if we conceive them to be tiausported to a lower ^^tude, 

 the yyeij^ht will press on the spring with diminished gfaJvijty.ailLd 

 the height of the column will be increased. - -t ^-rf-ma. ><. 



The oaflcHlations being iifiade iii the fir$t instance f^l^ttfe'fati- 

 tud^ of^5f,ti'e must /;/cre^^e the altitudes in latitudesi^feafBEthe 

 eqilatqri, and c////fJ///.sA those in parallels apprOachinjt^ the'i^ol^f^;^ in 

 th&i'atio* fH'opf^r ihv tlm'rcspective latitudes ai (!:tilculaled ty|f„t,jij^ 



