to the Antarctic Re2io7is, 219 



"fc. 



glass must be fixed a metallic wire terminating in a point, and con- 

 nected with an electroscope by means of a fine copper wire. If the 

 wire be made to terminate in a spiral wrapped round a piece of cotton 

 dipped in spirits of wine and inflamed, its power of collecting elec- 

 tricity will be sometimes doubled, but great precautions are neceS" 

 sary when this mode is employed. When the electroscope has been 

 charged, the nature of the electricity may be tested in the usual way 

 by excited glass or sealing wax. 



The principal electroscopes which are capable of being employed 

 to ascertain the electrical state of the atmosphere, or rather to com- 

 pare its state at any given elevation with the state of the medium in 

 contact with the instrument, are the following. 



1. De Saussure's electrometer, which consists of two fine wires, 

 each terminated by a small pith ball, and adapted to a small metal rod 

 fixed in the upper part of a square glass cover, upon one of the faces 

 of which a divided scale is marked, in order to measure the angles 

 of deviation of the two balls. 



2. Volta's electrometer, formed of two straws about 2 inches long 

 and :|;th of a line broad, suspended from two small very moveable 

 rings adapted to a metal rod : to measure the deviation of the straws 

 a telescope Avith a nonius is employed. 



3. Singer's electrometer, consisting of two slips of gold leaf sus- 

 pended from the rod. 



4. Bohnenberger's electroscope, formed of a single strip of gold 

 leaf suspended from the conducting rod between two dry piles, the 

 negative pole of one and the positive pole of the other being upper- 

 most : this arrangement has the advantage of indicating the kind of 

 electricity communicated to the conductor. 



The observations made with these and similar instruments have 

 demonstrated that in serene weather the electricity of the atmosphere 

 is always positive with regard to that of the earth, and that it be- 

 comes more and more positive in proportion to its elevation above 

 the earth's surface; so that if an observer be on a mountain or in a 

 balloon, if his conductor be directed downwards to reach an inferior 

 stratum of air, his electroscope will indicate negative electricity ; and 

 if it be sent upwards into a superior stratum, positive electricity will 

 be manifested. Various means have been resorted to in these ex- 

 periments, such as connecting one of the extremities of the conduct- 

 ing wire to a kite, a small balloon, or the head of an arrow, the 

 other extremity remaining attached to the electroscope. 



It has been ascertained by the observations of De Saussure, Schub- 

 ler, Arago and others, that the positive electricity of the atmosphere 

 is subject to diurnal variations of intensity, there being two maxima 

 and two minima during the twenty-four hours. The first minimum 

 takes place a little before the rising of the sun ; as it rises, the in- 

 tensity, at first gradually and then rapidly, increases, and arrives at 

 its first maximum a few hours after. This excess diminishes at first 

 rapidly and afterwards slowly, and arrives at its minimum some hours 

 before sunset ; it re-ascends when the sun approaches the horizon, 

 and attains its second maximum a few hours after, then diminishes 



