VOL. LI.] -^ PHILOSOPHICAL TRANSACTIONS. 505 



pass through the glass, but electrifies one side plus, and the other side minus, 

 as Dr. Franklin has shown in his letters on electricity. 



Hence he collects, that the three different effects, viz. the^ electrifying glass 

 plus on both sides ; or plus on one side, and minus on the other ; or, lastly, 

 minus on both sides, are occasioned by the different degrees of the same power 

 and resistance in the respective experiments with the same glass. 



Mr. W. concludes with an experiment made by Mr. Hamilton, professor of 

 philosophy in the university of Dublin, as it seems to illustrate the doctrine of 

 resistances, at least, so far as respects the air. 



Let a slender brass, or iron wire, 5 or 6 inches long, and finely pointed at 

 each end, be fitted in the middle, with a brass cap, void of angles ; then let 

 half an inch at each extremity be bent in opposite directions, till they are per- 

 pendicular to the rest of the wire, and in such a manner, that when the wire is 

 suspended, by means of its cap, on a point of metal, it may lie in a plane paral- 

 lel to the horizon. The pointed metal, which supports this wire, must be 2 or 

 3 inches long, and have its other end fixed into a small block of wood. Now, 

 if this block, with a wire suspended, be set upon an electrified body, the 

 wire will turn round with a very great velocity, moving always in a direction 

 contrary to that in which the electric fluid issues from its points, without hav- 

 ing any conducting substance near it, save that of the air : and if the wire be 

 made to turn round by any other force, in the opposite direction, so that its 

 points go foremost, it will when electrified, soon be deprived of that motion, 

 and be made to turn round the contrary way. 



This experiment, he says, was contrived, to try whether the electric fluid, 

 which issues so freely from pointed bodies, w'ould have any effect to move these 

 bodies by its reaction ; and that it has such an effect seems sufficiently ma- 

 nifest from the event. Mr. Hamilton apprehends, that the electric particles, 

 by their elastic force, issue directly forwards from the points, and endeavour to 

 expand themselves ; but meeting with some resistance from the air, force the 

 wire to move backward in a contrary direction, much in the same manner that 

 a Catherine-wheel is made to turn round in a direction, contrary to that in 

 which the small rockets affixed to its periphery discharge themselves. And 

 therefore, he is inclined to thipk, that it might be made use of as an electro- 

 meter, by having it to turn round in a plane perpendicular to the horizon, 

 and loading the wire with small weights near one of its extremities, which will 

 be raised to a greater distance from the perpendicular line, as the electric fluid 

 is stronger. 



yoL. XI. 3 T 



