106 



state of maximum saturation, or in proportion as the leaf can tran- 

 spire moisture through its cell walls. 



Therefore any ^observations of evaporation that we may make for 

 comparative purposes can give us only the relative evaporating 

 power of the wind and not the absolute evaporation from plants and 

 soils. 



THE PICHE EVAPORIMETER. 



The simplest apparatus for observing evaporation is that known 

 as the Piche evaporimeter. This consists of a glass tube closed at 

 the top and hung in a free exposure ; the tube is less than half an inch 

 in diameter and filled wdth Avater; its lower open end is closed by a 

 horizontal disk of bibulous paper about twice the diameter of the 

 tube; the water evaporated from this paper is supplied from within 

 the tube. The observer has simply to read the height of the water 

 in the tube as it slowly descends hour by hour. The number so 

 read oif is easily converted into one that expresses the depth of water 

 evaporated per hour from unit surface. 



The following table from Montsouris Annuaire, 1888, page 254, 

 shows the average evaporation thus determined by an instrument 

 placed in the shade, also the corresponding temperatures and other 

 data, as observed at Montsouris during thirteen years. 



Evaporation at Montsouris. 

 [Averages for 1873-1885.] 



Number 

 of days. 



Temper- 

 ature of 

 air in 



Tension 



of vapor 



in air. 



Relative 



hu- 

 midity. 



Hourly 

 velocity 

 of wind. 



Hourly 

 evapora- 

 tion. 



January . . . 

 February.. 



March 



April 



May 



June 



July 



August 



September 

 October . . . 

 November 

 December . 



3.6 

 6.0 

 9.1 

 12.? 

 16.3 

 20.0 

 22.0 

 21.6 

 17.6 

 12.5 

 8.0 

 3.6 



mm. 

 4.8 

 5.4 

 5.4 

 6.3 

 7.4 

 10.1 

 11.1 

 11.4 

 10.2 

 8.0 

 6.2 



Per cent. 

 80.9 

 77.0 



57.3 

 54.0 

 58.2 

 56.5 

 59.4 

 68.0 



77.5 



82.4 



Kilom. 

 15.9 

 16.1 

 17.8 

 17.6 

 17.5 

 15.3 

 14.7 

 15.7 

 14.4 

 15.4 

 18.1 

 15. 6 



mm. 

 0.084 

 .101 

 .187 



.254 

 .234 

 .154 



THOMAS RUSSELL S OBSERVATIONS. 



Prof. Thomas Russell, of the Signal Office, has published results 

 of some observations on the effect of the wind on the evaporation 

 from the disks of the Piche evaporimeter. (See Annual Report 

 Chief Signal Officer, 1888, p. 17(5, oi- Monthly Weather Review, 1888, 



