S40 On Governor Ellis's Discovery of the Action of Cold 



If the axis be diminished from the point where the weight acts 

 towards the pouits of support, so as to be of eqnal strength in 

 every part, the figure of each semi-axis will be titat generated 

 by the revohition of a cubic parabola. In this case the de- 

 flexion is greater ; being, when /■ = the greatest diameter, 



J^'i- = 8*. (C.) 



The same may be shown in the case of a hollow cylinder, and 

 it may be proved generally tlsat an uniform cylinder, whether 

 solid or hollow, is stifter than any figure inscribed within it. 

 When an axis is formed of cylinders of different diameters, 



the deflexion of the middle part will be ^^pUW '' ^ 

 deflexion of the extremities - — ^rr— - ; conseciuently the whole 



12;) Ai ;■' 



deflexion will be .j^ X (^^ + "1^)= ^- (^•) 

 Where L is the whole length, I the sum of the lengths of the 

 small cylinders, R the radius of the middle, and r the radius 

 of the small cylinders. If the two cylinders be of different 

 metals, M wilf differ in value in the different members of the 

 equation. [To be continued.] 



LXV. On Governor Ellis's Discovery of the Action of Cold 

 on Magnetic Needles. 



To the Editors of the Philosophical Magazine and Journal. 

 . Lansdown Crescent, Bath, Nov. 9, 1822. 



Gentlemen, — A few days ago, in looking over your pub- 

 lication for last September, my attention was arrested by part 

 of a letter from B. De Sanctis, M.D. " respecting some fri- 

 gorific experiments made on the magnetic fluid." Their prin- 

 cipal I'esult is, — that at certain low temperatures magnetic 

 needles lose their powers, which the writer attributes to the 

 action of cold on the magnetic fluid. Neither the fact, how- 

 ever, nor the explanation is new. Above seventy years ago 

 the fact was made known to the world, and a nearly similar 

 explanation f of it given by Mr. Ellis:]: in the account he pub- 

 lished 



* Essay on Cast-iron, artt. 81, 90, and 100. 



f The chief difference of the explanations is only this : Mr. Ellis con- 

 jectnred that the action of cold on the needle, as well as on the magnetic 

 particles or fluid, jnay contribute to produce the effect ; while Dr.De Sanctis 

 excludes the action of cold on the needle from any share of the effect. 

 " It is only, or at least principally," he says, " the action of cold on the 

 magnetic fluid itself that produces the paralysis" of the needle ; and he 

 intimates that tlie magnetic fluid mm/ be frozen into a refined etticrcal ice! 



\ The late Governor Henry Ellis, who died in IHOlJj and was, for se- 

 veral 



