288 The Plant World. 



The wax does not penetrate the papei but adheres firmly to its 

 outer surface. In setting up the instrament, the cylinder is 

 lowered over the seat till it comes into position, the water reser- 

 voir is raised till water drips from the lower edge of the paper, 

 and the wax cup is pressed firmly against the seat while the res- 

 ervoir is lowered. With proper care the reinforced cylinder tip 

 grips the tin seat with sufiicient force to maintain itself in position. 

 The base of the cylinder should be supported as above. 



In the paper cylinder atmometer, the evaporation takes 

 place almost entirely from the exterior of the cylindrical surface. 

 At the same time an ample air space separates the moist paper 

 from the water-filled metal tube, and very rapid adjustment to 

 changes in the surrounding temperature is allowed. For some 

 purposes it may be desirable completely to close the annular 

 opening below, as by a thin cork ring. The volume of water 

 held, while considerably greater (proportionately to the active sur- 

 face exposed) than in the thinner paper disk forms, is not large. 

 These thimbles hold water to an amount about equalling 70 per 

 cent, of their dry weight, this being less than the amount held 

 in the foliage of many^ thin-leaved plants. The surface may be 

 blacked by the use of washed lamp-black and water, as on the 

 porous-clay cup, for the measurement of light effects, and the 

 whole cydinder may be placed in any position, at any angle, with 

 perfect safety. It has been found that the process of washing 

 sometimes results in a more or less pronounced agglutination of 

 the lamp-black into larger granules, (at least in the case of cer- 

 tain varieties of this material) and a substitute therefor has 

 been found in the finely divided manganese dioxid which results 

 from the reduction of potassium permanganate by boiling its 

 solution with a little cane sugar. The precipitate is washed and 

 decanted several times and then used in the same manner as 

 lamp-black. 



In the perfecting and testing of the paper cylinder atmom- 

 eter I have been assisted by Mr. Lon A. Hawkins, to whose in- 

 genuity the details of the metal seat are especially due; also by 

 Prof. J. S. Caldwell, to whose able co-operation at the Desert 

 Laboratory during the summer just passed are due the working 

 out of the sealing-wax cap and the manganese dioxid coating. 



