EVAPORATION OF EVERGrvEEN AND DECIDUOUS TREES. 



239 



cemented also another short glass tube into the plate, 2 inches 

 long-, and 1 inch in diameter. Through this tube I watered the 

 plant, and then stopped it up with a cork ; I also stopped up the 

 hole at the bottom of the pot with a cork." 



Matters being thus arranged, the plant received a weighed 

 supply of water, and being itself weighed twice a day for a fort- 

 night, tiie rate of evaporation was easily observed. By another 

 comparative experiment Dr. Hales ascertained the quantity of 

 water evaporated every day by tiie porous earthen pot, and sub- 

 tracted it from the whole daily loss sustained by tlie Sunflower. 

 The result showed that on an average the plant evaporated 20 

 ounces or 34 cubic inches of water in a 12 hours day ; the maxi- 

 mum proportion being 30 ounces. This was certainly a very 

 interesting and remarkable experiment, and it was rendered all 

 the more so by the careful and minute details which accompanied 

 its publication ; including the bulk and length of the roots, and 

 the exact size of the leaves. Hales also measured the rate of 

 evaporation of a Cabbage, a Vine, a young Apple tree, and a 

 Lemon tree. Tlie result of these experiments is expressed in the 

 followinsr table: — 



Sunflower 

 Cabbage . 

 Vine . 

 Apple-tree 

 Lemon-tree 



Entire 



Sui't'ace of 



Leaves. 



Square inches. 

 5616 

 2736 

 1820 

 1589 

 2557 



Water 



Perspired in 



12 hours. 



Ounces. 



20 



19 



5i 



9' 



101 



Ratio of 



Evaporation 



to Surface 



of Plant. 





The practical conclusion drawn from tliese experiments was, 

 that the Cabbage evaporated the greatest quantity of water, and 

 the Lemon tree the least. On repeating it with other plants Dr. 

 Hales found that in all cases evergreens perspired less than those 

 plants which shed their leaves in the winter, a fact which he endea- 

 voured to explain by observing, that " as they perspire less, so 

 they are better able to survive the winter's cold." At the same 

 time that he made these experiments he also made a number of 

 other highly interesting ones, on the force with which plants 

 absorb water, and many similar points connected with tliis part 

 of their economy. 



A third series of experiments was made by Mr. Miller in 

 1726, at the Chelsea Botanic Garden, at the suggestion of Dr. 

 Hales, in which the subjects of experiment were a Musa, an Aloe, 

 and an Apple tree. The plants were growing in glazed earthen- 



