SOIL MOISTURE AND TO EVAPORATION. 25 



sensitive to slight variations from hour to hour, due to air currents, etc. , 

 for, as soon as the water surface falls below the upper edge of the con- 

 taining vessel, this surface is protected, to some extent at least, from 

 the full action of the wind. In attempting to relate plant transpira- 

 tion to physical evaporation it became necessary to devise a form of 

 evaporimeter which should, if possible, be as sensitive both to vari- 

 ations in air currents and to those in relative humidity as the plant 

 itself. At the same time it should be capable of giving readings for 

 short periods of time, so that changes in the rate of evaporation from 

 minute to minute and from hour to hour might be studied. 



Happily, a method was hit upon, which, while it gives practically 

 perfect results, is exceedingly easy of operation and requires a minimum 

 of time and care. The apparatus consists essentially of one of the 

 unglazed porcelain cylinders described on page 20, closed by a rubber 

 stopper carrying a glass tube, the opposite end of which is connected 

 with the outlet of a burette. When the cylinder is placed considerably 

 above the level of the top of the burette and the whole apparatus is 

 filled with water, the pressure of the air is entirely removed from the 

 water in the cylinder, since the water films across the capillary pores 

 of this porcelain will support at least one atmosphere of air pressure, 

 and thus the liquid fails to flow down into the burette. At the same 

 time, evaporation of water from the surface of the moist porcelain is 

 constantly accompanied by a corresponding outward seepage from within, 

 and therefore also by a corresponding withdrawal from the burette. 

 Evaporation from the meniscus of the burette column is prevented either 

 by an oil layer, as shown in figure 4, or by a nearly air-tight closure of 

 the top of the burette, using an inverted test tube or a cork stopper 

 with a small opening cut in one side. The last method is most satis- 

 factory. Readings are taken from time to time of the contents of the 

 burette, and the difference between any two readings gives the volume 

 of water lost from the evaporimeter surface for the period of time 

 intervening between these readings. Only distilled water should be 

 used, since the gradual accumulation of salts within and on the walls of 

 the cylinder alters appreciably the rate of evaporation and thereby 

 introduces an error into the record. 



For ease in filling the burette its inlet tube was connected with an 

 elevated separatory funnel, so that it could be refilled at will by simply 

 opening the cock at the base of the latter. The whole apparatus may be 

 mounted on a ring-stand so as to be easily portable (see fig. 4), or the 

 evaporimeter tube may be fixed permanently out of doors and the 

 burette and reservoir may stand in a room, the two parts of the 

 instrument being connected by a tube which passes through the wall. 



