Homogeneous Bodies; wifh illustrative Experiments. 123 



made in precisely the same manner as those described with 

 masses of zinc ; and the results, excepting in degree, were ex- 

 actly the same in both metals. The thermo-magnetic energies 

 were very promptly and uniformly displayed in this mass of 

 copper, but were exceedingly feeble when compared to those 

 developed by a mass of zinc less than half its size. With 

 the latter metal, a needle, on which the terrestrial magnetic 

 powers were in full play, could be made to sweep an arch of 

 100°; whilst with the unwieldy mass of copper, it required 

 the soliciting terrestrial force to be entirely cut off from the 

 needle in order to obtain a sweep of 6° or 8°. 



115. There does not seem to be that uniformity in the dis- 

 play of thermo-magnetism by thin metallic plates as is ob- 

 served to be developed by those of considerable thickness. 

 The phaenomena, when thin plates are employed, although 

 the metal be neither rolled nor hammered, assume a very ca- 

 pricious character, and appear to be governed by laws which 

 are not easily traced to any general standard. 



116. I am not at present prepared to say to what cause 

 these phaenomena are attributable: they seem to be of a 

 distinct order, and not referable to the laws of crystallization. 

 They may possibly be traced to a difference in the progress 

 of heat in the several parts of the metal, moving with dif- 

 ferent degrees of celerity in the margin and body or area of 

 the mass. Should this conjecture be correct (and I have some 

 reasons to think that it is true), I imagine that this class of 

 experiments will exhibit a very prominent feature amongst all 

 those, which, from time to time have been advanced for the 

 solution of the highly important problem of terrestrial mag- 

 netism, more particularly in that branch of the inquiry which 

 relates to the diurnal variation. 



117. Experimeiits 'with Spheres of Zinc. — To carry the ana- 

 logy of experiment still closer to terrestrial magnetic action, I 

 have had cast, globes of zinc of different sizes, with a view of de- 

 tecting some law by which their thermo-magnetic energies are 

 exhibited when heat is partially distributed over the surface of 

 the sphere, in imitation of the sun's action on the face of the 

 earth. One of these globes is solid, and about 5'54i inches 

 diameter, weighing nearly 23 pounds; another, which is 

 hollow, and 10 inches diameter, weighs 64 pounds, the thick- 

 ness of the metal being about '15 inch. 



118. With these spheres I have as yet gained but little 

 information, owing, as I suppose, to the difficulty which I have 

 experienced in keeping the various parts of their surfaces at 

 temperatures sufficiently remote from each other. 1 have, 

 however, succeded in deflecting a needle by applying to it the 



R 2 10-inch 



