58 Transactions of the SoutJi African Pliilosopliical Society. 



De Saussure found the surface of some of the Swiss lakes more 

 than 30° warmer, in summer, than the bottom. Symons mentions 

 occasional differences of 10° to 15° between the surface and bottom 

 of the Strathfield Turgiss tank in June and July — the warmer water 

 being at the top of course. Therefore, in the daytime at any rate, 

 the temperature of the surface has not only to assist in the process 

 of evaporation, but it has also to assist in raising the temperature of 

 the next lower stratum. So that it is not at all evident (has any one 

 ever attempted to prove ?) that a body of water heated from below will 

 lose the same quantity by evaporation, so-called, as another equal body 

 heated naturally from above, even though the surface temperatures 

 be the same. There is another consideration : In heating the w^ater 

 from below, the heat of the stove, or of the sides of the vessel, will 

 generate an upcast of warmer air whose humidity ratio will not be 

 that of the air outside the laboratory, but something much less ; 

 consequently the readings of the dry and wet bulbs, meteorologically 

 speaking, do not matter. These remarks refer more particularly to 

 the experiments described in the few original papers on the subject 

 it has been my good (or evil) fortune to have seen. 



Halley has described the results of an experiment upon the rate 

 of evaporation in which the water was allowed to evaporate without 

 first being artificially heated. Unfortunately the results are scarcely 

 more than a laboratory product, for the gauge was shielded from both 

 sun and wind. His conclusions, nevertheless, were, as usual, pre- 

 eminently sagacious: "I caused an experiment to be made," says 

 the note, '* of the quantity of vapour arising simply from the warmth 

 of the water, without being exposed either to sun or wind, which has 

 been performed with great care and accuracy by Mr. Hunt, operator 

 to the Society : having added together the evaporations of the whole 

 year, I find that from a surface, as near as could be measured, of 

 8 square inches there evaporated during the year 16,292 grains of 

 water, which is 64 cube inches of water, and that divided by 8 inches,, 

 the area of the water surface, shows that the depth of water evaporated 

 in one year amounts to 8 inches. But this is much too little to 

 answer to the experiments of the French, who found that it rained 



in these days of refrigerating machinery. He made several trials with a bucket 

 sea-gauge in lat, 25' 18" N., long. 25' 12" W,, letting it down to different depths 

 from 8(50 to 5,84(5 feet. He discovered by means of a small thermometer of 

 Fahrenheit, which went down with it, that the cold increased regularly, in propor- 

 tion to the depths, to 8,900 feet, where the temperature was 53° or a little less ; the 

 temperature of the air and of the ocean-surface being 84". It is remarked that the 

 experiments, " which seemed at first but mere food for curiosity," afterwards proved 

 very useful. By its means tliey supplied their cold bath, and cooled their wines 

 and water at pleasure — " which was vastly agreeable in that burning climate." 



