288 



The great velocity with which electricity travels in conducting 

 bodies having been urged as an objection to this explanation, the 

 author combats this objection, on the ground that small differences 

 of electricity ought (from analogy with a fluid similarly circum- 

 stanced,) to be equalized more slowly than great ones ; and more- 

 over, that by the disposition of the apparatus, extremely weak forces 

 being made to act constantly and for a long time, ought to produce 

 an effect much superior to that arising from the transient action of 

 each part. 



The author next proceeds to examine the action of screens inter- 

 posed ; and having first satisfied himself by direct experiments with 

 unelectrified metallic plates and needles, that currents of air driven 

 through a screen of muslin produced no sensible effects, he placed 

 a screen of coarse gauze between an electrified sealing-wax needle 

 and a revolving pewter plate, and to his surprise found a tendency to 

 motion in the needle, opposite to that of the plate ; and though not 

 always produced, nor always to the same extent, this effect occurred 

 in seventeen out of twenty trials. 



The author then looked out for some method of increasing the in- 

 equality of distribution of electricity on the metallic plate ; he there- 

 fore placed on the revolving apparatus, just under the metal plate, a 

 very small lighted lamp. Several experiments made with this dispo- 

 sition of the apparatus are related, without leading to any conclusion 

 as to the action exerted on the needle ; and the author then proceeds 

 to consider what extraneous causes could have acted to produce the 

 small retrograde motions observed. First, such causes are enume- 

 rated as currents of air in the room, in the box containing the appa- 

 ratus ; currents driven through the screen by rotation ; currents of 

 heated air from the lamp ; vibrations from the mechanism producing 

 the motion ; torsion of the wire suspending, or electricity of the bridge 

 carrying, the needle ; and flexure of the wax. 



Before relating the experiments made to elucidate each of these 

 disturbing causes, however, he proceeds to relate other experiments, 

 confirmatory of the fact of the retrograde motion, and of the influence 

 of the heat in producing it ; after which, he describes a great variety 

 of experiments, made for the purpose of trying the effect of the pre- 

 sence or absence of the disturbing causes ; and concludes that none 

 of them, singly or combined, are adequate to produce the phenomena 

 observed. 



He then proceeds to state what appears to him to be their true 

 explanation, on the principles adopted respecting the non-instanta- 

 neous communication of electricity ; or at least to show that they are 

 not repugnant to those principles ; or that, moreover, those principles 

 may in certain cases give rise to a retrograde motion, or to no mo- 

 tion at all, or a direct one, according to the disposition of the appa- 

 ratus ; and that very trifling apparent differences in the latter respect 

 may give rise to all the varieties of the phenomena. 



