OF ARTS AND SCIENCES. 459 



The sphere can be raised and lowered by moans of clockwork, an 

 electric motor, or a water motor. The laboratory has a small water 

 motor, which was used for this purpose by attaching an arm to the 

 circumference of the wheel and fastening the end of the string to this 

 arm. When the arm is in a certain position of its revolution, the 

 sphere rests lightly on the bottom of the cup. When the arm has 

 turned 180° from this position the sphere touches the plate. 



The sphere, cup, and plate must be of the same metal. As the 

 quadrants of the electrometer are of brass, we made these of brass to 

 avoid all contact electricity. 



Comparative observations have been made with this collector and 

 the water-dropper for a month. The changes seem to be similar, but 

 the deflections of the water-dropper are the larger. 



It seems possible, with some mechanical improvements, to make 

 this form of collector superior to any other. 



Observations. 



The observations show that — 



The potential of the air was generally low and positive, seldom as 

 high as 25 or 30 volts. 



The potential usually fell before precipitation, storms, or when the 

 relative humidity increased. 



The potential during precipitation, with a very few exceptions, was 

 always low and positive. 



Almost all the negative electricity, except that which was followed 

 by precipitation, occurred during west to northwest gales, or during 

 cold waves. 



Low clouds sometimes seemed to affect the observations, but high 

 clouds seemed to have no influence. 



There was very slight variation with altitude, — at least, between 

 two and ten meters above the ground. 



There was no apprecial)le variation between collectors placed on 

 different sides of the buildinfj- 



& 



Ox Obtaining the Electric Potential of the Upper Air. 



On the morning of May Cth, the potential of the air at a point ten 

 feet above the ground and three feet from the walls of the laboratory, 

 obtained by the usual water-dropping method, was, reduced to volts, 

 0.5. A paper kite covered with cloth and tinfoil, with its longest axis 

 about four feet, was flown, the connecting string being heavy English 



