MANUAL or THE NILAGTI^T DISTRICT. 69 



as powerful here as at the level of the sea_, but the heat that CHAP. IV, 

 composes it is modified by free radiation. PAET III. 



The mean annual range and the mean daily range of temper- Wellington. 

 ature are small, the warmest month being May with the mean Medical 

 temperature of 66*4, and the coldest month December with the Climate, &c. 

 mean temperature of 57*5. The mean annual height of the 

 barometer uncorrected is 24*246, and the mean daily variation of 

 the radiation thermometer is 25"2. The respective readings of 

 the sun^s maximum in vacuo between this and the level of the 

 sea shows that the diminished heat of the earth and atmosphere 

 of elevated regions is not due to any less quantity of solar 

 influence received, but to the latter being radiated away into 

 space, almost as fast as it is received, and, if it were not for the 

 difference in the amount of obscuration of the sun between 

 Madras and Wellington, the reading of the thermometer showing 

 the sun's maximum in vacuo would be exactly the same. 

 Between the maximum thermometer fully exposed at Wellington 

 and that at Madras there is a difference of 21° Fahrenheit, which 

 is the amount of heat lost by radiation, between this and the 

 level of the sea, and here, as is usual near the equator, the annual 

 and daily range of the barometer is not very great. In May 

 1872 the barometer fell to 24'018, but this was contemporaneous 

 with the cyclone which devastated the Madras roads; also, on 

 the 6th May 1874, during a cyclone at Madras, the barometer at 

 Wellington fell to 24*052. Rain, which is one of the most 

 important elements in determining climate, amounts annually, 

 on an average, to 46* 75 inches. Here it has small influence 

 on the barometer, and the popular idea that the fall of the baro- 

 meter indicates rain is a fallacy. It represents irregular currents 

 of air forming wind. This is notorious in the tropics, where the 

 wind is mostly ranged into uniform trades or monsoons, and rain 

 is almost unnoticed by the barometer. A very simple rule for 

 ascertaining heights of mountains and mean temperatures of such 

 places may be noticed here. If we are able to obtain the pressure 

 of the barometer, which is easily done by a pocket aneroid, and 

 knowing that for every 1,000 feet of ascent the barometer falls 

 very nealy one inch, we find that the pressure of the barometer 

 at Wellington is 24 inches, whilst the reading at the level of 

 the sea is always calculated at 30, which shows a less pressure 

 of six inches, making its elevation 6,000 feet, which is very nearly 

 correct. In the same way, if we admit that the mean tempera- 

 ture at the sea is to be calculated at 83° Fahrenheit, and for every 

 300 feet of ascent the thermometer falls a degree, we may 

 calculate the mean temperature, if we know the height of a locality. 

 Thus Wellington is looked on as 6,000 feet above the sea. Divide 

 300 into 6,000, and you will get 20 as the result. Deduct this 20^ 

 from the temperature at the level of the sea, viz., 83 degrees, and 



