200 ^ff^<^i of Cold on the Magnetic Fluid, 



that the magnetic foixe augments in vacuo tlirough the abstrac- 

 tion of the resistance of tlie air. 



Now, sir, I am thoroughly convinced of the exactness of the 

 indication, if not of the graduation, cf the Httle watch-needle 

 I had the honour of showing you at your lectures at the Lon- 

 don Institution. That needle, amongst many other interesting 

 facts, in the course of my last magnetic researches presented to 

 me some time ago the same effects when plunged in a bath of 

 ice and muriate of lime ; but to avoid the objection that the 

 movements of such a needle were not to be depended upon, 

 and that they might be paralysed by the dampness of the air 

 at so low a temperature, I wished to repeat the experiments 

 at large, and in a vacxumi, where certainly there was no damp- 

 ness till the return of the air caused the vapours of liquefied 

 ice which penetrated through the grease to fall upon the pa- 

 rallel opipeds, the needles and their supports — where a small 

 degree of humidity was afterwards sensible, but only to the 

 touch. This objection however at present can no longer exist, 

 since I repeated at Mr. Gary's the same experiment on a 

 larger needle similar to the lesser one above described. It 

 was placed in a wooden case, likewise covered with glass, and 

 protected from any effect of dampness by spreading the ground 

 with dry powdered muriate of lime ; and it was really inter- 

 esting to see how soon the needle passed from activity to slug- 

 gishness at the alternate immersions and emersions from the 

 trigorific mixture. 



Amongst the many principles of refonn in the construction 

 of magnetic needles, of which I spoke to you, as being partly 

 tried and partly to be tried, Mr. Schmalcalder agrees with 

 me, that it would be perhaps better to destroy the action of the 

 magnetic forces, in order to balance the inclination needle, by 

 intense cold than by intense heat. The steel is always da- 

 maged by a high temperature, notwithstanding every precau- 

 tion ; and besides, I have seen practically enough in these mat- 

 ters to be able to assert, that even the incandescing heat can- 

 not entirely destroy the magnetic power of any considerable 

 mass of steel or iron, excepting loadstone of some particular 

 mines ; and even wei'e it to happen momentarily in thin laminar 

 steel, as soon as it cools the power begins to reappear. I was 

 communicating the new project to Mr. Garden, of Oxford- 

 street, when he very intelligently remarked that it would per- 

 hajis be better to try it in vacuo, as the numidity necessarily 

 arising from a too low temperatm-e in the air, might disturb 

 the exact effect of gravity by its imequal precipitation on the 

 needle and the augmentation of friction. In fact, that which 

 was scarcely observable, even after the readmission of the air 



in. 



i 



