358 WRITINGS OF JOSEPH HENRY. [1855- 



twenty-four hours, correspond inversely with the variations 

 in the amount of vapor. 



The experiments we have thus far described are intended 

 to establish the inductive character of the atmosphere in its 

 condition of dryness and serenity, particularly during clear 

 and cold weather. 



We have employed movable conductors terminated by 

 balls which have been of the most favorable form and rela- 

 tive dimensions to exhibit the effects of induction. The 

 apparatus usually employed before the experiments of 

 Peltier, were principally stationary insulated conductors 

 terminated by points above, which as we have seen act 

 powerfully in discharging electricity from a body, or in ab- 

 sorbing it from the surrounding medium. 



If in the experiments with the apparatus, Fig. 16, the rod 

 be terminated by a point instead of a ball, but feeble excita- 

 tion will be observed during clear, cold weather, because 

 the point exhibits so exceedingly small a surface that but 

 very little electricity can be drawn down into the lower end 

 before the intensity of attraction of un-saturated matter up- 

 wards comes into an equilibrium with the attraction of the 

 earth downwards. With this instrument the observer would 

 probably make a record to the effect that the electricity of 

 the atmosphere was very feeble, whereas if the experiment 

 were made with the apparatus previously described an 

 opposite condition would be noted. But the result would 

 be entirely different if the air were damp, and the insulated 

 rod elevated to a considerable height: the negative intensity 

 of the upper end would be sufficient to attract a portion of 

 the natural electricity from the surrounding medium, even 

 although this had become slightly negative by the previous 

 induction of the earth. In this case the pointed conductor 

 would indicate a large amount of electricity. 



The intensity of the induction may even become so great 

 as to absorb a portion of the natural electricity of the dry 

 atmosphere as in the case of a very long wire, the upper end 

 of which is furnished with a series of points, and raised to a 

 great height by means of a kite. The points may attract 



