132 Analysis of Scientific Books. 



between zero and the greatest separation of the images without 

 exchanging any part of the eye-tube, it being only required to 

 move tlie axis in which the sphere is placed. It also possesses 

 the property of an eye-tube or lens not intended for microme- 

 trical purposes, for when the axis of the crystal is parallel to 

 that of the object glass, only one image will be formed. A 

 plate is annexed to this paper, illustrative of the contri- 

 vance. 



X. The Bakerian Lecture : on the best kind of Steel and Form for 

 a comjjass-needle. By Captain Henry Kater, F.R.S. 



On the return of the expedition under the command of 

 Captain Ross, which sailed in search of a north-west passage, 

 it appeared that the compasses became nearly useless from the 

 diminution of directive force, in consequence of the vicinity 

 of the magnetic pole. Some of the azimuth compasses then 

 employed were of Captain Kater's invention ; he was therefore 

 naturally anxious that the next expedition, (under the command 

 of Lieutenant Parry,) should be furnished with instrtmients 

 combining as much power and sensibility as possible ; with this 

 intention, our author undertook the experiments which form the 

 subject of the valuable practical document before us, and from 

 which he draws the following inferences : 



" 1. That the best material for rompass-needlcs 13 chck spring; but care 

 must be taken in forming the needle to expose it as seldom as possible 

 to heiit, otherwise its capability of receiving magnetism will be much 

 diminished. 



" 2. That the best form for a compass-needle is the pierced rhombus, in the 

 proportion of al)out live inches in length to two inches in vridth, this fonn 

 being susceptible of the greatest directive force. 



" 3. That the best mode of tempering a compass-needle is, first to harden 

 it at a red heat, and then to soften it from the middle to about an inch from 

 each extremity, by exposing it to a heat sufficient to cause the blue colour 

 which arises again to cfisappear. 



" 4. That in the same plate of steel of the size of a few square inches 

 only, portions are found varying considerably in their capability of receiving 

 magnetism, thourfi not apparently ditfering in any other respect. 



" 5. That polisliingthe needle has no effect on its magnetism. 



" 6. That the best mode of communicating magnetism to a needle, 

 appears to be by placing it in the magnetic meridian, joining the opposite 

 poles of a pair of bar magnets (the magnets being in the same line), and 

 laying the magnets so joined flat upon the needle with their poles upon its 

 centre ; then having elevated the tiistant extremities of the magnets, so that 

 they may form an angle of about two or three degrees vnth the needle, they 

 are to be" drawn from" the centre of the needle to the extremities, carefully 

 preserving the same inclination, and having joined the poles of the magnets 

 at a distance from the needle, the operation is to be repeated ten or twelve 

 times on each surface. 



" 7. That in needles from five to eight inches in length, their weights 

 being equal, the directive forces are nearly as the lengths. 



" 8. Thiit the directive force does not depend upon extent of surface, but 

 in needles of nearly the same length and form, is as the mass. 



" 9. That the deviation of a compass needle occasioned by the attraction 



