312 PHILOSOPHICAL TRANSACTIONS. [anNO 1772. 



The electrometer placed out from a garret window has been frequently useful, 

 in determining the nature of an approaching cloud, whose electricity, though 

 generally strong, was for the most part uncertain, having been sometimes posi- 

 tive, and at other times negative. But as the wind or rain were frequent impe- 

 diments to the accuracy of the experiments, the following methods of making 

 observations with success, under shelter, occurred. Mr. R. has sometimes stood 

 in an upper room, on a cake of wax, holding in his right hand, out at the 

 window, a long slender piece of wood, round which a wire projecting a few 

 inches had been twisted, and in his left hand an electrometer; an assistant had 

 excited glass or wax in readiness. At other times he made use of a tapering 

 tube of tin, 20 feet long, ending in a point; the greatest part of it stood out 

 high in the air, and the thick end, from which an electrometer hung, was sup- 

 ported inside the window, sometimes with silk cords, and at other times with 

 strong sticks of sealing wax, sustained at either end by hooks of iron wire. By 

 either of these means he often discovered, that what seemed a single cloud, pro- 

 duced, in its passing over, several successive changes, from positive to negative 

 and from negative to positive electricity, the balls coming together each time, 

 and remaining in contact a few seconds, before they repelled each other again. 



The permanence of either kind of electricity in the clouds, or the length of 

 time in which neither can be discovered, is uncertain; sometimes the same elec- 

 tricity has returned, and at other times has been succeeded by the contrary ; 

 while either generally came on, and went off gradually. But changes were 

 often made, very suddenly, by a flash of lightning, especially if the thunder- 

 storm happened to be in the zenith. A branch of it over head, has frequently 

 occasioned stronger electricity than he could discover, when the greatest part of 

 the sky had been overcast; which perhaps might be accounted for from this con- 

 sideration, that one kind of electricity acting alone, must exert more powerful 

 effects than when counteracted by the other. 



He once observed in a thunder-storm, during which he saw no lightning, 

 that the balls, which hung from the tin tube, repelled and attracted each other, 

 very rapidly, for the space of 10 or 12 seconds; at the same time Mr. Canton's 

 electrometer, which he held at such a distance from the tube, as to have its 

 balls opened to the distance of an inch, continued quiet in that state, and were 

 not affected convulsively like the others. Hence he imagined, that the same 

 kind of electricity went off, and came on, without being changed in contrarium; 



pears by Dr. Franklin's curious experiment with the chain and silver can. I also have discovered from 

 repeated trials, that a piece of flannel, silk, &c. excited, and suddenly twisted, not only struck at a 

 greater distance than before, but sometimes emitted pencils of fire into the air. May we not hence 

 infer why the electricity of vapour, &c. (when not in contact with the eartli) increases by condensa- 

 tion ? — Orig. 



