194 Prof. Moseley's Reply to Mr. Earnshaw's 



nished us with facts, which, though not completely established 

 as belonging to any peculiar class of phsenomena, and there- 

 fore not generally admitted into systematic works, are not the 

 less worthy of notice. Those which bear most directly on 

 our present speculation were observed by M. Fresnel*; namely, 

 the repulsion of disks of mica, of which one was placed at the 

 extremity of a delicately suspended needle in vacuo, and when 

 the disks were in contact, heated by means of a ray of solar 

 light concentrated by a lens. M. Saigeyf has also described 

 a class of similar phsenomena observed by himself, with a con- 

 siderable number of metals, which after rejecting the in- 

 fluence of aerial currents, of electricity, magnetism, &c, he 

 ascribes to the repulsive action of radiant heat at sensible di- 

 stances. There are several other experiments on record which 

 seem to require a similar explanation, but I apprehend that 

 the present are the first to establish the existence of some 

 species of mechanical repulsion in the propagation of heat, a 

 principle which can hardly fail to be applicable to the expla- 

 nation of many natural phsenomena. 

 Greenhill, Edinburgh, Feb. 19, 1833. 



XXXIV. Reply to Mr. Earnshaw's Remarks on the "Principle 

 of Least Pressure." By the Rev. H. Moseley, B.A., Pro- 

 fessor of Natural Philosophy in King's College, London. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



TN submitting to the consideration of Mr. Earnshaw the 

 -*- following answer to his remarks on my paper purporting 

 to contain the demonstration of a new statical principle, which 

 I have called that of least pressure ; I beg to suggest to that 

 gentleman that the obscurity of which he complains is perhaps 

 in some respect attributable to the nature of the subject under 

 discussion. The theory of statical resistances has always been 

 considered among the most difficult and complicated of those 

 subjects which lie within the range of exact science, yielding 

 itself most unwillingly and ungraciously to the control of ana- 

 lysis, and such, in its nature, as cannot be submitted to the test 

 of experiment. 



It is nevertheless a subject entering, more or less, into the 

 complete discussion of almost every question \ that can occur 



* Annates de C/iimie et de Physique, xxix. 57. and 107. 

 f See several successive articles in the Bulletin des Sciences Mathema- 

 tiques, torn. ix. See also Pouillet, Elemens de Physique. 



I There is practically scarcely any case of equilibrium, among the forces 



