5 io THE POPULAR SCIENCE MONTHLY. 



be accomplished by receiving a spot of light, which is reflected from 

 the mirror attached to the needle, upon a cylinder covered with sensi- 

 tized paper. This cylinder is made to slowly revolve by clock-work, 

 so that twenty-four-hour observations can be obtained. Nothing is so 

 characteristic of modern methods in physical science as continuous 

 registration of physical phenomena. Early observers were forced to 

 content themselves with scattered observations taken at different in- 

 tervals, and important variations might occur at the times when no 

 observer was watching the apparatus. Now the movements of the 

 human heart can be recorded by a little apparatus which will combine 

 in one curve a thousand continuous observations, and the slightest 

 anomalous fluctuations can be studied from the record which is ob- 

 tained. 



Provided, therefore, with these modern and more refined means of 

 observation, we are in a condition to study the fluctuations of that 

 subtile manifestation of energy which is ever present in the air, and 

 seems to lurk in all matter. The influence of the electrical state of the 

 air upon atmospheric changes is doubtless far-reaching. We are accus- 

 tomed to think of thunder-storms as the only manifestations of atmos- 

 pheric electricity ; but there are influences which it exerts, more silent 

 than those which are announced by the crackle of lightning, yet none 

 the less extended. A pretty experiment, described by Lord Rayleigh, 

 illustrates the effect of electricity upon the coalescence of rain-drops. 

 A narrow stream of water is allowed to issue from a reservoir and form 

 a parabola, which strikes the ground in drops about four feet from the 

 orifice. When this is a fine stream, it breaks up into a shower of drops 

 at about two feet from the orifice. On rubbing a bit of sealing-wax 

 with a cloth, and presenting the rod of sealing-wax near the stream, 

 the drops immediately cease to separate, and the stream is continuous 

 from the orifice to the point where it strikes the ground. This is a 

 very striking experiment, and undoubtedly has a bearing upon the 

 formation of large drops of rain, which are often noticed during a 

 thunder-storm after a discharge of lightning. Instead of a rod of 

 sealing-wax, a piece of writing-paper can be readily charged by plac- 

 ing it upon a table and rubbing it vigorously with the palm of the 

 hand. 



None of the theories which endeavor to explain the source of the 

 electricity of the air are satisfactory. Professor Tait, in a recent 

 lecture, is of the opinion that evaporation is the source. Under the 

 influence of the sun's heat, this operation of nature is conducted on an 

 immense scale, and seems to him to be competent to furnish the supply 

 of atmospheric electricity. 



It is maintained by others that the atmosphere of the earth has a 

 permanent charge, which it received in the beginning, or which was 

 developed by cosmical changes ; and this charge manifests itself here 

 and there, and fluctuates under different conditions of heat and moist- 



