524 Cannon : A method of measuring transpiration 



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in summer not infrequently registers 95° F, (ss"" C) in the shade, 

 and often much higher. The weight of a cubic meter of saturated 

 air at 35° C. is 39.187 grams; the maximum vapor-pressure at 

 that temperature is, however, 4178 mm.* I have, therefore, used 

 the hygrometric tables in making all calculations. 



If, now, the absolute humidity is found for the above experi- 

 ment by employing the hygrometric tables we shall get slightly 



different results. 



The weight of one cubic meter of aqueous vapor at 29"^ C, 

 and at sea-level, is given as 28.450 grams.f With a relative 

 humidity of 2^ per cent the weight would be 6.54 grams for one 

 cubic meter. Multiplying this by the fraction 0.0175, as before, 

 gives 114 milligrams for the first reading. The second amount 

 reduced in the same manner is 248.8 milligrams. Therefore the 

 total transpiration, according to the latter method of making the 

 computation, is 134.8 milligrams as against 142 by the first 

 method, and this difference becomes greater with the rise of the 

 temperature, since, among other reasons given above, the first 

 method was based upon the pressure of vapor and not on its 

 weight. 



It has been mentioned above that the hygrometric tables are 

 computed for sea-level. For delicate hygrometric work, there- 

 fore, some correction should be made for the higher altitudes, but 

 for the usual work of estimating the transpiration of plants, partly 

 because it is largely a comparison, slight altitudes above the sea 

 need not be considered. Neither is it necessary to take into 

 account the usual daily variation of the barometer. When on 

 account of a considerable altitude above the sea, as at the Desert 

 Botanical Laboratory (altitude 2,600 feet), it is necessary to take 

 into account the diminished air-pressure, the final deductions for 

 absolute humidity are multiplied by the following fraction : 



Gravity at altitude of experiment 



Gravity at sea-level 



♦Smithsonian Meteorological Tables, 1897, pages 133 and 142 

 fZ. r., page 133. 



