Intelligence and Miscellaneous Articles, 327 



ocular demonstration of the cause of the changes in the apparent" 

 size of the moon. 



A stereoscopic slide, copied from the figure referred to, forms 

 Xo. 27 of the white-lined diagrams upon a black ground, published 

 in Paris soon after the appearance of the article in the Philosophical 

 Magazine. 



In poirjting out the value -of the stereoscope in the investigation 

 of difficult and important questions of a physical and metaphysical 

 nature, Sir David Brewster has referred more fully to this experi- 

 ment in his ' Treatise on the Stereoscope,' pp. 200-204, published 

 in 1S56. 



I am, Gentlemen, 



Yours, &c, 



September 13, 1S62. Alpha. 



HISTORY OF THE DYNAMIC THEORY OF HEAT. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen - , 



Mr. Joule, in his " Note on the History of the Dynamical Theory 

 of Heat," mentions Locke as the first who regarded heat as motion ; 

 but I am inclined to] think that Lord Bacon has a prior claim to the 

 theory. 



In the 20th Aphorism of Book II. of the Novum Gryanum he says*, 

 "The nature whose limit is heat appears to be motion. This is 

 chiefly exhibited in flame which is in constant motion, and in warm 

 and boiling liquids which are likewise in constant motion. It is also 

 shown bv the excitement or increase of heat bv motion as by bellows 

 and draughts, . . .. . . and by the extinction of fire and heat upon any 



strong pressure which restrains and puts a stop to motion." Among 

 these far-fetched and quite inapplicable examples we find one which 

 is used in the most recent text-books. Other species of motion 

 produce heat, he says, and refers the reader to Aph. 13, Xo. 31 : — 

 " The anvil becomes so hot by the hammer, that if it icere a thin plate 

 it might probably grow red, like ignited iron, by repeated strokes." 



Bacon defines heat as follows, "heat is not a uniform 



expansive motion of the whole, but of the small particles of the 

 body ; and this motion being at the time restrained, repulsed, and 

 reflected, becomes alternating, perpetually hurrying, striving, strug- 

 gling, and irritated by the repercussion, which is the source of the 

 violence of flame and heat." 



Bacon's inquiry concerning the nature of heat has been condemned. 

 It is true that his reasoning is in many points at fault, that his exam- 

 ples are badly chosen, and often do not apply : but when we con- 

 sider the amount of physical knowledge in the world in Bacon's 

 time, and that this was the first attempt at arranging together all 

 the properties of a force and inferring the nature of that force from 

 its properties, and, lastly, that Bacon came to the conclusion that 

 heat is a kind of motion, a theory now generally accepted, — when we 



* According to J. Devev's translation. 



