64 C LIMA-IE. 



heights not exceeding two miles, the difference is 1° of Fah-^ 

 renheit for every 300 feet of ascent. 



Now the mean annual temperature at the level of the 

 sea in the latitude of the hills, according to the formulae 

 of Mayer, as corrected by Playfair, is 83°*, and the height 

 of Ootacamund, above the level of the sea, being 7,416 feet, 

 the calculation will be as follows '^-^^z=.2^^.^2' the theoretic 

 difference of temperature between the level of the sea and 

 Ootacamund, consequently 83° — 24°.72'==58°.28, mean an, 

 nual temperature of Ootacamund. And it will be seen here- 

 after that the annual temperature of Ootacamund, deduced 

 from the mean of 25 months' observations, is 58o.68, so that 

 the difference between this, the observed mean, and that de- 

 duced from theory, is only 0^.40, which, trifling as it is, is 

 perhaps to be accounted for by the fact of the observations 

 alluded to not being for consecutive months : the mean for 

 some of the colder months being wanting. 



The next most obvious effect of the elevation of these 

 regions is, the increased power of the sun's direct rays, in 

 proportion as the conducting power of the medium is dimin- 

 ished ; in other words, as the air is rarefied. This effect is 

 further increased by the smaller absorption of heat in its 

 passage to the surface, the thickness of the stratum of air 

 through which the sun's rays have to pass, being diminished 

 by the amount of the elevation above the level of the se^. 

 No exact formulae for calculating this effect of the sun's 

 rays have yet been given, owing partly to the want of good 

 photometrical observation^, and the number of disturbing 

 causes to be taken into account. 



That such is the fact will, however, be evident from con^ 

 suiting the annexed meteorological tables, where it will be 

 * iPrewpter's Edin. Encyclop. Article Meteorology, 



