458 THE POPULAR SCIENCE MONTHLY. 



Collecting Apparatus. — Dellman's apparatus for collecting atmos- 

 pheric electricity is a hollow brass or copper ball about six inches in 

 diameter, with a stem of metal. The metallic stem rests in a metallic 

 tube, but is isolated from the tube by shellac. This apparatus is at- 

 tached to a pole almost thirty feet long. This pole is drawn by a 

 windlass up the walls of a house to the top of the roof. The operator 

 then touches the stem of the ball with a piece of brass in the shape of 

 a half-moon. This charges the ball with electricity. The pole is now 

 let down at once, and the collecting apparatus is brought in control 

 with the measuring apparatus. The electricity which is thus collected 

 in the ball is developed in it by induction. The natural electricity of 

 the ball is separated by the surrounding atmospheric electricity into 

 positive and negative electricity. One of these goes to the lower part 

 of the ball, the other remains in the upper part. Atmospheric elec- 

 tricity is usually positive. The natural electricity of the ball being 

 decomposed, the negative is attracted to the upper and the positive to 

 the lower part. When the operator touches the stem of the ball with 

 the piece of brass, the positive electricity is conducted through his 

 body to the ground, and the negative remains in the ball. 



When, therefore, the electrometer shows negative electricity, it indi^ 

 cates positive electricity in the atmosphere, and vice versa. 



It has been shown that there are two daily tides of positive atmos- 

 pheric electricity — the high tides between 9 and 12 a. m. and be- 

 tween 6 and 9 p.m.; the low tides between 2 and 5 p. m. and 1 and 5 

 a. m. The annual variations are fully as marked as the diurnal ; the 

 quantity of positive atmospheric electricity being greatest in the win- 

 ter, least in the summer. Dr. Wislizenus found that, in 2,124 obser- 

 vations made at regular hours, the atmospheric electricity was 2,046 

 times positive and but 78 times negative. Of the 78 times, 30 were 

 connected with thunder or hail storms, or by thunder and lightning, 

 23 by common rains, and 20 by high winds and gales without rain, 

 thunder, or lightning, 4 by snow, and 1 by fog. 1 



According to Herschel, out of 10,500 observations at the Royal 

 Observatory, only 364 showed negative electricity. The remainder, 

 10,176, all showed positive electricity. Negative electricity was usu- 

 ally attended with rain. 



It seems, therefore, that the chief cause of a condition of negative 

 atmospheric electricity is storm, and especially thunder-storms, and 

 that at all other times positive atmospheric electricity prevails. In 

 very many cases this change to negative electricity takes place shortly 

 before the storm approaches ; during its progress there may be — espe- 

 cially in thunder-storms — rapidly-repeated alternations of positive and 

 negative conditions, followed by an equilibrium, or by positive elec- 

 tricity. 



Dr. Wislizenus, of St. Louis, also found that snow-storms and fog 



1 Dr. A. Wislizenus, in Transactions of St. Louis Academy of Medicine. 



