220 Instructions fur the Scientific Expedition 



till sunrise, and proceeds in the order already indicated. The in- 

 tensity of the free electricity of the atmosphere has also been found 

 to undergo annual changes, increasing from the month of July to 

 the month of November inclusive, so that the greatest intensity 

 occurs in winter, and the least in summer. 



In cloudy weather the free electricity of the atmosphere is still 

 positive. During storms, or when it rains or snows, the electricity 

 is sometimes positive and sometimes negative, and its intensity is 

 always much more considerable than in serene weather. The elec- 

 troscope will, during the continuance of a storm, frequently indicate 

 several changes, from positive to negative. 



The above is a short summary of almost all that is known respecting 

 the laws of atmospheric electricity. It will be highly important to 

 obtain a series of observations equal in accuracy to those made by 

 Schubler at Frankfort in 1811 and 1812, simultaneously with the 

 observations of the hygrometer, barometer, thermometer, &c. Com- 

 bined observations at a number of different stations cannot fail to 

 give us important information respecting the distribution of the free 

 electricity in the atmosphere, and the extent and nature of the dis- 

 turbances to which it is subject ; but to render the results valuable 

 it will be necessary to have instruments comparable with each other, 

 and this niay be a difficult matter to effect*. 



Very recently a new method of investigating the electric state of 

 the atmosphere has been proposed, likely to lead hereafter to very 

 certain and valuable results ; but it has not been sufficiently put in 

 practice to enable the Council to recommend, at the present mo- 

 ment, the best form of instrument for making simultaneous and com- 

 parable observations, or the proper precautions to guide the observer 

 in manipulating it. 



For the principle of this instrument we are indebted to Mr. Col- 

 ladon of Geneva. He found, that if the two ends of the wire of a 

 galvanic multiplier, consisting of very numerous coils well insulated 

 from each other, were brought in contact, one with a body positively, 

 and the other with a body negatively charged, a current of electricity 

 })asses through the wire, until equilibrium is restored ; the energy and 

 <lirection of this current is indicated by the deviation of the needle 

 from the zero-point of the scale. This instrument is applied to the 

 purpose of ascertaining and measuring the atmospheric electricity, 

 by communicating one end of the wire with the earth, and allowing 

 the other to extend into the region of the atmosphere, the electrical 

 state of which is intended to be compared. 



Thunder storms, of course, should be attended to ; but it is of con- 

 sequence also to notice distant lightning not accompanied with 

 thunder audible at the place of observation, especially if it take place 

 many days in succession, and to note the quarter of the horizon 

 where it appears, and the extent which it embraces. In an actual 

 thunder storm, especial notice should be taken of the quantity of rain 



* For a fuller account of what is known respecting atmospheric electricity, and 

 the mode of conducting the observations, see Bccquerel's Traite de V Ekctricite, 

 t. iv. pp. 78—125. 



