14 



THOPICAL NATURE, AND OTHER ESSAYS. 



day, till the increased pressure caused the leakage to 

 increase so as exactly to balance the supply ; while in 

 the other the water will sink till the decreasing pressure 

 causes the leakage to decrease so as to balance the 

 supply, when both will remain stationary, the one at 

 a high the other at a low average level, each rising 

 during the day and sinking again at night. Just the 

 same thing occurs with that great heat-reservoir the 

 earth, whose actual temperature at any spot will depend, 

 not alone upon the quantity of heat it receives, but on 

 the balance between its constantly varying waste and 

 supply. We can thus understand how it is that, although 

 in the months of June and July Scotland in latitude 

 57° north receives as much sun-heat as Angola or 

 Timor in latitude 10" south, and for a much greater 

 number of hours daily, yet in the latter the mean 

 temperature ayIII be about 80° Fahr., with a daily 

 maximum of 90° to 95°, while in the former the mean 

 will be about 60° Fahr. with a daily maximum of 70° or 

 75° ; and, while in Scotland exposure to the full noon- 

 day sun produces no unpleasant heat-sensations, a similar 

 exposure in Timor at any time between 9 a.m. and 3 

 P.M. would blister the skin in a few minutes almost as 

 effectually as the application of scalding water. 



Heat Due to the Condensation of Atmospheric 

 Vapour. — Another cause which tends to keep up a 

 uniform high temperature in the equatorial, as compared 

 with the variable temperatures of the extra-tropical 

 zones, is the large amount of heat liberated during the 

 condensation of the aqueous vapour of the atmosphere 

 in the form of rain and dew. Owing to the frequent 

 near approach of the equatorial atmosphere to the 



