1-16? The Plant World. 



efficient with its date. The table of losses from the several cups 

 in operation shows the cup numbers as well, so that reference 

 to the coefficient record slips gives all the necessary data for 

 correction when the results are to be calculated. 



Reduction of corrected readings to depth. — Evaporation has 

 been measured like rainfall, in units of depth, centimeters or 

 inches. • This m.ethod has arisen from the desire to give evapora- 

 tion rates in terms of loss in depth from lakes, ponds, etc. But 

 of course the soil varies to a marked degree in its power to give 

 off water, this power depending on the water content, the fine- 

 ness of the soil, the vegetational cover, etc. Thus the records in 

 inches or centirreters are supposed only to denote an hypo- 

 theticallv possible evaporation, if the whole area considered were 

 a water surface. However, even this does not approach the 

 truth; the depth, the kind of sediment and other factors enter 

 into the phenomenon of natural evaporation from water bodies. 

 Also, it has long been known that the form, size and material 

 of an evaporating pan exercise a great influence on the rate of 

 evaporation. Therefore, if this climatic factor is measured in 

 terms of depth it is necessary that the water surface referred 

 to be accurately described. It has often been suggested that 

 the readings of the porous cup atmomieter should be reduced to 

 centimeters or inches of evaporation. This can be done, but 

 onlv with reference to a m.ore or less arbitrarily chosen water 

 surface. This point is clearly brought out by the following con- 

 siderations. The standard cup loses one gram for each 0.2217 

 mm. in depth of the standard pan, which contains approximately 

 the volume of water held in the cup and has a surface approach- 

 ing that of the uncoated instrument. This is a very small vol- 

 ume with a relatively large surface. Comparison with a hemi- 

 spherical copper pan about 25 cm. in diameter and 12 cm. deep 

 gives 0.1416 mm. in depth as corresponding to one gram from 

 the standard cup. Comparison with one of the two-foot pans in use 

 at the Salton evaporation station of the U. S. Weather Bureau 

 (the operation of which I was able to study for several days in 

 June, 1909, through the kindness of Prof. Frank Bigelow, in 

 charge of this work), gives 0.137 mm. as equivalent to a gram 

 from the standard porous cup. Thus we see, it is only through a 

 standaird pan or tank, arbitrarily chosen, that units of depth 



