530 TRANS. OF THE ACAD. OF SCIENCE. 



changed into high negative. The negative and positive signs 

 are sometimes changing quick in succession during a thun- 

 derstorm, but the prevailing electricity is rather the nega- 

 tive. 



In high winds without rain or thunderstorm (dry storms) 

 the atmospheric electricity changes often into high negative, 

 and, when accompanied by a sudden fall of the barometer, it 

 is a sure indication of a storm running over a large extent of 

 territory. Such storms, originating here in the west or 

 southwest, travel generally in a semi-circular curve to the 

 northwest and northeast, and reach the Atlantic coast in 

 about twenty-four hours. By means of the electrometer and 

 barometer such storms may be predicted and telegraphed to 

 the sea coast a day beforehand. But for want of a meteoro- 

 logical centre such forewarning is lost to the public. In Eng- 

 land, France, Prussia, and Italy, they have established me- 

 teorological boards to make their observations useful to the 

 public by sending telegraphic warnings to the threatened 

 points. If in countries of so small geographical extent benefit 

 is derived from such an institution, how much greater would 

 be the advantage in a country like ours, of continental ex- 

 pansion? 



Bain without thunderstorm is as often accompanied by 

 positive as negative electricity, but still oftener by no elec- 

 tricity at all, because in continued rains atmospheric electri- 

 city is fast carried to the earth. 



Snowing is generally accompanied by high positive electri- 

 city. The quantity of positive electricity developed in a 

 snowfall is often so great that it will create a vacuum of the 

 usual positive electricity for a great distance from the snow- 

 fall. When, therefore, in winter, in cold, dry weather, with 

 clear sky, the usually high positive electricity disappears sud- 

 denly, it is an indication that in a distance (of hundreds of 

 miles) snow falls in considerable masses. 



Fog is always accompanied by high positive electricity. 



The regular positive electricity of the atmosphere exhibits 

 a three-fold periodicity : 



1. A daily one (see table 3), with two maxima and two 

 minima every day; the first maximum appearing about 9 

 o'clock in the morning, the second about 6 in the evening, 

 and the first minimum appearing about 3 in the afternoon, 

 the second about 9, and still later in the night. 



2. A monthly periodicity (see table 1) electricity being 

 higher in the colder months, and lower in the warmer months 

 of the year. 



3. A yearly periodicity (see the yearly mean of electricity 

 in table 1). From 1801-1803 the yearly mean of electricity 

 was ascending, while from 1864-1867 it is gradually descend- 

 ing, so that the mean of 1867 is only half the quantity of that 



