i CLIMATE AND ASPECTS OF THE EQUATORIAL ZONE 227 



voirs are both leaky, but while the one loses at the rate of. 

 nine hundred gallons in the twenty-four hours, the other loses 

 at the rate of eleven hundred gallons in the same time, sup- 

 posing that both are kept exactly half full and thus subjected 

 to the same uniform water-pressure. If now both are left to 

 be supplied by the above-mentioned pipes the result will be, 

 that in the one which loses by leakage less than it receives 

 the water will rise day by day till the increased pressure 

 causes the leakage to increase to such an extent as exactly to 

 balance the supply ; while in the other the water will sink till 

 the decreasing pressure causes the leakage to decrease till it 

 also just balances 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 under- 

 stand 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 countries 

 the mean temperature will 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 AtmospJieric Vapour 



Another cause which tends to keep up a uniform high tem- 

 perature 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 atmos- 

 phere to the saturation point, and the great amount of vapour 

 its high temperature enables it to hold in suspension, a very 



