-1859] 



WRITINGS OF JOSEPH HENRY. 



851 



ing the balloon — was insulated. The experiments should be 

 repeated at different elevations by some of our modern 

 aeronauts, since the results obtained would have an impor- 

 tant bearing on the theory of atmospheric electricity. 



The same results may be shown in a simpler manner by 

 the method invented by Saussure. This consists in attaching 

 a leaden ball/, (Fig. 12,) to a long wire covered with silk or var- 

 nish, connected by means of a slight spring to the hook of 



an electroscope. When this bulb is 

 thrown upward by means of a string 

 and handle p, so as to rise to a con- 

 siderable height in the air, the pith 

 hsXlsg g, of the electroscope diverge 

 with positive electricity, and the 

 wire is dis-connected from the in- 

 strument. That this effect is not due 

 to the friction of the bulb and the air 

 is shown by whirling it in a hori- 

 zontal circle round the head ; not 

 the least sign of electricity in this 

 case being exhibited: and that it 

 is not charged by absorbing free 

 electricity from the air, is proved by the fact that when 

 the ball is thrown horizontally no excitement is manifest. 

 The result is however just such as would be produced by the 

 induction of the earth acting on the natural electricity of the 

 wire and drawing it down to its lower extremity. A pre- 

 cisely similar effect would also be produced if the upper 

 surface of the atmosphere were charged with this electricity. 

 The intensity of the charge which the electroscope receives 

 wiir depend upon the elevation to which the ball ascends, or 

 in other words on the perpendicular component of the direc- 

 tion of the wire. 



The method employed by Saussure in observing the varia- 

 tions of the electricity of the atmosphere illustrates the same 

 principle. For this purpose he made use of one of his own 

 electroscopes such as shown in Fig. 12. It consists of a 

 bell-glass with a brass stem,, d e, surrounded with sealing- 

 wax, and two small pith balls, g g, suspended by very 



Fig. 12. 



