Radiation from the Earth's Surface. 



405 



In beginning these experiments it was found that the 

 effect of care in the preparation of the instrument was to 

 greatly decrease the ordinary rate of collapse of the leaves. 



Fi^. 1. 



After repeated trials the following method was adopted as 

 D'i^-ino- the most satisfactory and consistent results. The 

 three brass sections were first carefully polished with metal- 

 polish, then washed with soap and hot water, and thoroughly 

 dried with a clean towel. The glass dish containing the 

 drying agent was similarly washed and dried, fresh calcium 

 chloride placed in it, and then coyered with perforated tin- 

 foil, pre^-iously cleaned with ammonia. The sulphur bead 

 was made larger than required, and then pared down with a 

 sharp clean knife, the utmost care being taken during this 

 operation not to touch the bead with the fingers. All the 

 joints and the edges of the mica windows were then carefully 

 waxed oyer, and the instrument set aside for a day to give 

 the calcium chloride time to dry the inclosed air, and render 

 the insulation of the sulphur bead as perfect as possible. 

 The leakage of electricity across the sulphur bead could 



