298 Notices respecting New Boohs. 



are supported on the glass tube r 3 which is fastened by the cap s 

 on to the plate-glass disk t, which rests upon the top of the 

 glass cylinder u clamped upon the wooden base c resting on 

 levelling-screws. In the side of u is a plate-glass window, w, 

 through which a vertical line of light may be focused upon x (a 

 mirror fastened to the mml system), and thence thrown upon a 

 scale in the manner which is now so often employed. 



A word or two about the way in which the instrument is used. 

 The upper plate t and the system m ml are removed by lifting r. 

 The edge of u is rubbed with beeswax to prevent t from slipping 

 upon it. The copper wires penetrating the cups are amalgamated 

 and a little mercury poured in. Amalgamated thin platinum- 

 foil is then pressed into the cups, and mercury is poured upon 

 this. By this means a concave meniscus is obtained. The upper 

 partis then replaced, and so adjusted by turning the plate / and 

 the cylinder u that the mirror x is parallel to the window w, 

 when the axis of mm 1 makes an angle of about 15° with that 

 of// 7 . The rod / is adjusted so that the wires of m ml just touch 

 the mercury ; and by the levelling-screws k is so swung that m 

 and/, and also ml and/', are exactly opposite to one another and 

 the wires in the centres of the mercury-cups. A slit of light is 

 then sent through w, reflected on to a screen, and the head o is 

 then turned till the slit is split by an arbitrary vertical line on 

 the screen. The reading of p is then noted. A current passing 

 through the system forces m ml away from //'. Turn the head 

 o until the slit of light is again brought to the mark on the screen. 

 The angle through which it must be turned is directly propor- 

 tional to the magneto-repulsion — that is, to the square of the 

 current-strength. Many of the laws of electrodynamics may be 

 readily illustrated by this instrument ; and not only may differ- 

 ent currents be compared with the greatest accuracy, but the 

 absolute mechanical magneto-value of the current may be at 

 once arrived at. By bringing the repellent magnets always to 

 the same distance from one another, a whole class of sources of 

 error is removed. 



XLIII. Notices respecting New Boohs. 



First Lessons in Theoretical Mechanics. By the Rev. John F. 

 Twisden", M.A., Professor of Mathematics in the Staff College, 

 and formerly Scholar of Trinity College, Cambridge. London : 

 Longmans, Green and Co. 1874: pp. 243. 



HPHOSE teachers and students w T ho are already acquainted with 

 -*■ the author's large Treatise on Mechanics, will naturally ex- 

 pect tofind in the work now before us perfect exactitude both in 



