Septembeb 4, 190S] 



'SCIENCE 



319 



TLe writer would also value information 

 about any nepheline rocks in Massachusetts. 



B. K. Emerson 

 Amherst, Mass. 



a simple atmometer 

 For determining the differences between 

 the evaporating power of the air in different 

 localities, as in the case of studies dealing 

 with the relation of meteorological conditions 

 to plant growth, the atmometer here de- 

 scribed has proved very satisfactory. This 

 instrument utilizes a porous clay bougie for 



Fia. 1. Porous Cup Atmometer 



the evaporating surface, after the manner of 

 an atmometer devised by Babinet and de- 

 scribed in 1848.' It is a modification of a 

 form independently devised by the author for 

 physiological purposes, and described in Pub- 

 lication No. 50 of the Carnegie Institution, 

 1906. 



The bougie is about 13 cm. long and 2.5 cm. 

 in diameter, closed and rounded at one end 

 and reiniorced at the other by a thickened 

 rim. The wall, of unglazed porcelain similar 

 to that used for filter tubes, is about 4 mm. 

 in thickness. The open end is closed by a 

 perforated rubber stopper bearing a glass tube 

 about 30 cm. in length, which extends through 

 a cork stopper nearly to the bottom of a glass 

 jar of the " Mason " pattern. Any bottle will 

 serve as well, and a graduated fiask serves 

 better, but the " Mason " jar was adopted be- 

 cause of the ease with which it may be ob- 

 tained almost anywhere in the United States. 

 To allow access of air to the jar, the cork 

 stopper should fit the latter somewhat imper- 

 fectly, or should have a slight groove cut in 

 its margin. Above the jar the tube passes 

 through a conical cap of cloth which is ren- 

 dered water-proof by means of shellac. This 

 serves to shed rain water and to prevent its 

 direct entrance to the jar. An external file- 

 mark on the jar, near the shoulder (0 in the 

 figure), serves as a fixed water-level. A pint, 

 quart or half-gallon jar is used, according to 

 the evaporation rate and the time period dur- 

 ing which the instrument is to operate with- 

 out refilling. 



In setting up this instrument, the jar is 

 partially filled with distilled water, the bougie 

 (which has been soaked in distilled water to 

 remove air) is filled and its stopper inserted 

 with the glass tube, the tube is filled, and its 

 free end quickly thrust to the bottom of the 

 jar. In the last operation air must not enter 

 the tube. The jar is next filled to the file- 

 mark, the cork stopper placed in position, and 

 the instrument is ready for operation. When 

 the apparatus is thus arranged, the water 

 films closing the pores at the outer surface of 

 the bougie possess a tensile strength adequate 



> Babinet, J., Compt. Rend., 27: 529-30, 1848. 



