236 Mr. R. Phillips on the Magnetism of Pewter Coils* / 



lines of equal magnetic intensity, follows at once from refer- 

 ring terrestrial magnetism to the internal heat of the earth; 

 for where the climate is coolest, the difference of temperature 

 will be the greatest, and there the electric developments will be 

 more powerful. This explanation of the cause of terrestrial 

 magnetism is so far independent of the hypothesis used to ac- 

 count for the magnetism of the coils, that if the magnetism of 

 the coils proceeds from the direct conversion of heat into mag- 

 netic force, then terrestrial magnetism results from a similar 

 transfoimation. 



173. On the specific inductive capacity of cloud. — The tin pipe 

 was arranged before the brass jet of the boiler as in (83.), but 

 with the longer arm pointing to the zenith ; and in the longer 

 arm of the tin pipe, and concentric with it, stood a copper tube 

 having a cork inserted at each end ; the upper one of these corks 

 received the end of a glass tube, which soon after leaving the 

 cork turned off at a right angle, and this horizontal portion of 

 the glass tube was put in a tube-holder and thus supported and 

 insulated the copper tube ; finally, a copper wire passed through 

 the whole length of the glass tube to the copper tube ; and con- 

 sequently, on bringing the knob of a charged Ley den jar into 

 contact with one end of the wire, the copper tube became elec- 

 trified inductrically, and the tin pipe inducteously. The copper 

 tube was externally -^ inch diameter, and it extended a distance 

 of 7 inches into the tin pipe. 



174. A large Ley den jar was now charged positively by a plate 

 machine, and then the knob of the jar was supported against the 

 end of the copper wire. The tin pipe was presently connected 

 with an electrometer, and the electricity was carefully drawn off 

 from the copper pipe by means of a point which I held in my 

 hand, until the electricity passed but very slowly between the 

 copper and the tin pipes ; the tin pipe was now put in commu- 

 nication with the single-leaf electrometer, and the electric in- 

 tensity of the Leyden jar still further reduced if the electricity 

 passed too rapidly. The intensity with which I preferred work- 

 ing being that at which the positive electricity nearly ceased to 

 pass to the tin pipe; consequently, the electrometer connected 

 with the tin pipe, after being discharged, would soon indicate a 

 feeble positive charge. The cock of the boiler was now opened so 

 that the issuing steam might leave the boiler positive ; the only 

 effect of which on the electrometer was a feeble negative action, 

 produced by the negative steam-cloud acting inductrically on the 

 tin pipe. 



175. It occasionally happened, from circumstances which I 

 could not succeed in deteimining, that the steam-cloud re-esta- 

 blished a communication with the copper and tin pipes ; and so 



