282 The Plant' World. 



ture lag of the cup is sometimes more pronounced than that of 

 a leaf; as the air cools at sunset or is warmed at sunrise the large 

 mass of water in the cup keeps the exposed surface warmer or 

 cooler for a time, as the case may be. The coming and going of 

 clouds sometimes brings out similar failure of the instruments 

 quickly to respond to changes in the external temperature. A 

 leaf, on the other hand, responds to many such changes almost 

 instantly. Thus, for detailed analyses of conditions the instru- 

 ments as hitherto constructed are sometimes inadequate, and 

 the results may be misleading. 



In seeking a practical modification of the apparatus, by 

 which this defect might be remedied, my attention was first 

 drawn to the Piche atmometer *. This valuable device con- 

 sists essentially (see Fig. 1) of a graduated glass tube, closed 

 above and covered below with a circle of absorbent paper, the 

 latter having a pin-hole in its center. The tube is filled with 

 distilled water, the paper disk applied and affixed by an ade- 

 quate clamp, and the whole inverted. The entire disk soon be- 

 comes wet, and evaporation therefrom draws water from the 

 tube, air rising through the pin-hole to replace the water with- 

 drawn. The central portion of the disk is covered above by the 

 end of the reservoir-tube, below by a small plate which sup- 

 ports the paper. 



This instrument possesses little temperature lag; a paper 

 may be used which ii thinner than many comm.on leaves and 

 holds but little water, and the lag-effect of the water mass above 

 its central portion, and of the thin metal disk below, is negligible, 

 so far as I have been able to detect. But the Piche atmometei 

 has four chaiacteristics which makes it unsuitable for detailed 

 transpiration studies: (1) the hydrostatic pressure upon the 

 paper is irregularly variable; the entrance of air through the 

 pin-hole is exceedingly spasmodic, so that a considerable error 

 is introduced into small readings. (2) It is practically impos- 

 sible to place the evaporating portion at a distance from the 

 graduated reservoir, an arrangement which is often desirable or 

 necessary when the moist surface must be in close proximity 



*Piche, A. — Note sur L'atmismometre, Instrument Destinee a Measurer L 'evaporation. 

 Bui. Assoc. Sci. de France, IQ: 166-167, 1872. For other references see Livingston, Grace 

 J., An Annotated Bibliography of Evaporation, Mo. Weather Rev., 1908-1909. 



