Mechanical Science. 405 



point, I wished to see if I should obtain analogous effects from the 

 light of a candle and from that of the moon. At the end of three- 

 quarters of an hour I obtained a slight degree of magnetism in a 

 needle exposed to the violet ray from the light of a candle ; but 

 that from the moon had no effect. I must say, that when I expe- 

 rimented upon the lunar ray, the temperature did not exceed + 5°R. 

 When I shall have repeated the experiment in another season, I will 

 publish the result. 



From what is above stated, I am convinced that philosophers 

 operating in the way pointed out will find magnetism developed 

 by the ray, and for which neither the climate of Italy nor that of 

 England is required, but only the precautions stated above. They 

 will also perceive that the magnetism thus obtained is not tempo- 

 rary but permanent, as I have ascertained, by finding that at the 

 end of eight months my wires and needles were still magnetic. — 

 Pavia, April 10, 1829.— Bi hi. de Geneve. 



17.— Dr. Arnott's Natural Philosophy. — (To the Editor of the 

 Quarterly Journal.) 



11 Sir,— The public attention has been arrested by the statement 

 in Dr. Arnott's new work on heat, that power is derivable from 

 expanded air, at one fourth of the expense by which it is obtained 

 from the conversion of water into steam. 



" Dr. Arnott's inference is founded on an estimate of the quantity 

 of heat absorbed in the production of one cubic foot of steam, and 

 of that required to double the elastic force of five cubic feet of air. 

 The result is deduced from the ordinary tables of specific and latent 

 heats, and of the consequent developement of elasticity. The cal- 

 culations are, I believe, correct as far as they go ; but they neglect, 

 I think, one most important consideration. 



H It is proposed to heat a certain quantity of air in a certain 

 vessel, from common temperature up to 500°, and this air so heated 

 and expanded, having done its work, is to be exchanged for a fresh 

 supply of cold air, to undergo the same process. But the vessel 

 in which the air was heated to 500° will be itself equally heated, 

 and will communicate the heat to the fresh entering air ; it follows, 

 therefore, that a full supply of cold dense air will not enter spon- 

 taneously as Dr. Arnott assumes ; it must be forced in by a pump, 

 or the vessel must be allowed to cool ; if the pump be employed, 

 the power required to fill the vessel will be at least one-half of 

 that derived from the expansion ; if the cooling process be adopted, 

 we revert to the systems of Savery and Newcomen, in which the 

 evaporation of a few ounces of water was accompanied by the 

 heating and cooling of some hundred pounds of metal. 



" No difficulty of this sort exists in the modem steam-engine, 

 owing to that valuable property which allows matter in two differ- 

 ent states, and requiring enormously different quantities of heat, to 

 co-exist in the same place. Two contiguous quantities of differ- 



