M'Ci.i;li-and and Gilmuui{ — The iAcctric Charge on Rain. 27 



numbers have not been i-.icludeil in the table. "We think, however, that tlie 

 above table ia fairly re[ire>ientative of the .-snow ami hail which fell, except 

 that on one or two occasions large snowHakes were positively charged, thougli 

 it is pretty evident that the charge on snow was generally negative. Large 

 hailstones were always positively charged, and small hailstones negatively. 

 These small hailstones are about the usual size of raindrops, and generally 

 colourless. The only change in the form of precipitation which did not 

 cause a change in tlie sign of the charge was on March 27, when hail 

 appeared to change to large raindrops without any alteration either in the 

 sign or magnitude of the charge. It will be observed that the charge per c.c, 

 is much larger in the case of snow and hail than in the case of rain. 



Size of Drops. 



In order to find whether the sign or magnitude of the charge on the rain 

 is influenced by the size of the raindrops, some measurements of the latter 

 were undertaken. Work has been done on the sizes of raindrops by 

 Bentleyi and Defant." Bentley computed the sizes from the flour-pellets 

 formed by allowing the raindrops to fall into Hour spread on a tray. The 

 method adopted by Defant, viz., Weisner's, consisted in receiving the drops 

 on filter paper, and allowing them to spread. lu the present case the latter 

 method was employed. 



A mixture of one part of eosin to at least thirty of talc powder was 

 rubbed into the filter paper. When a drop of water fell on this, it left a 

 permanent pink circular stain as far as it spread. The relation between the 

 volume of the drop and the diameter of the stain w-as found by allowing 

 drops of known volume to fall on the filter paper, and measuring the stain 

 produced. At first it was thought that drops as small as raindrops could be 

 got from glass tubing drawn to a very fine point, and dipped in paraifin wax 

 to prevent the water from wetting the glass. On trial it was found that the 

 vast majority of raindrops were smaller than the smallest drops obtained in 

 this way. Spraying water was then tried, but the number of drops falling 

 on a given small portion of the area sprayed over was too variable. 



The method finally employed was as follows : — The water was allowed to 

 drop at constant pressure from a glass tube drawn to a very fine point, which 

 was dipped in paraffin wax. This tube was enclosed in an outer tube, open 

 at the lower end, through which a steady blast of air was driven by a com- 

 pression pump. The blast forced the drops from the end of the inner tube 



' Monthly Weather Review. Uctober, 1904. 

 Akad. Wiss. Wien, Sits: Ber. May, 1905. 



