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i CLIMATE AND ASPECTS OF THE EQUATORIAL ZONE 227voirs are both leaky, but while the one loses at the rate of.nine hundred gallons in the twenty-four hours, the other losesat the rate of eleven hundred gallons in the same time, supposingthat both are kept exactly half full and thus subjectedto the same uniform water-pressure. If now both are left tobe supplied by the above-mentioned pipes the result will be,that in the one which loses by leakage less than it receivesthe water will rise day by day till the increased pressurecauses the leakage to increase to such an extent as exactly tobalance the supply while in the other the water will sink till;the decreasing pressure causes the leakage to decrease till italso just balances the supply, when both will remain stationary,the one at a high the other at a low average level, each risingduring the day and sinking again at night. Just the samething occurs with that great heat-reservoir the earth, whoseactual temperature at any spot will depend, not alone uponthe quantity of heat it receives, but on the balance betweenits constantly varying waste and supply. We can thus understandhow it is that, although in the months of June andJuly, Scotland in latitude 57 north receives as much sunheatas Angola or Timor in latitude 10 south, and for a muchgreater number of hours daily, yet in the latter countriesthe mean temperature will be about 80 Fahr., with a dailymaximum of 90 to 95, while in the former the mean will beabout 60 Fahr., with a dailymaximum of 70 or 75 ; and,while in Scotland exposure to the full noon-day sun producesno unpleasant heat-sensations, a similar exposure in Timor atany time between 9 A.M. and 3 P.M. would blister the skin in afew minutes almost as effectually as the applicationof scaldingwater.Heat due to the Condensation of AtmospJieric VapourAnother cause which tends to keep up a uniform high temperaturein the equatorial, as compared with the variabletemperatures of the extra-tropical zones, is the large amountof heat liberated during the condensation of the aqueousvapour of the atmosphere in the form of rain and dew.Owing to the frequent near approach of the equatorial atmosphereto the saturation point, and the great amount of vapourits high temperature enables it to hold in suspension, a very