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 Atmospheric 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 
