Dr. Henry's Modified Disinfecting Apparatus. 205 



which is natural to the northern regions of the earth. The two 

 horizontal needleswiUdeviatefromtheirformerpositions having 



their north ends drawn towards the sphere : the central need e 

 being placed on the equator will remain undisturbed. It he 

 north pole of the sphere be now gradually turned upward., the 

 horizontal needles will be directed still more towards this pole, 

 and the north end of the central needle will incline. Ihe 

 ancrles of deviation and dip will increase more and more as 

 the north pole of the sphere advances towards the zenith; and 

 when it has arrived at that point, those angles ^vlll have arrived 

 at their maximum. The central needle wil stand vertically, 

 exhibiting a dip of 90° over the pole ; and the two horizontal 

 needles will point almost directly towards .t. By Permitting 

 the sphere to resume its first position as gradually as it left it, 

 the needles which are influenced by it will be observed to re- 

 cede as gradually to their former positions. ,,,,(. 

 Ifnow the galvanic connections be reversed, the whole ot 

 the needles become reversed also, because of a change in the 

 polarity of the sphere; and the dipping needle, after a iew 

 oscillations, will settle in a horizontal position. 



By experimenting in this way with various positions ot the 

 spheVe, it will be found to operate on the needle m a manner 

 highly imitative of the earth's magnetism on different parts ot 



its surface. 



The two polar regions of the sphere exhibit a diffused pola- 

 rity, the centres of which are nearly, perhaps exactly, in the 

 poles of its equator. There are, however, in the polar regions 

 several points which exhibit distinct polarity; and although ot 

 the same character as the general pole m which they are situ- 

 ated, will draw a delicate needle held near to them, from he 

 direction which it takes when held at a greater distance from the 

 action of the general or aggregate pole of that P^'-J^^^^^/J/;"' 

 These local poles, I imagine, arise from a want of unifoimity in 

 the character of the iron. There are on the earth's surface abei- 

 rations of this kind arising from the local attractions of certain 

 islands, and from causes not easily determined ; which, as it 

 happens, these accidental poles may serve to imitate. 



XXVI. Letter from Dr. Henry on a Modified Disinfecting 

 Apparatus. 

 To the Editors of the Philosophical Magazine and Annals. 



IT waf my intention to have postponed any further com- 

 A munication respecting disinfection, until 'V'^'-ies of exper.- 

 monts, which 1 had hoped would, ere this, have been made 



