138 Notices respecting New Boohs, 



portion of the nickel tongue 1*5 centim., width 0*8 centim., 

 thickness 0*08 centim. The end of the nickel is filed to a blunt 

 point (about 135°). A stout copper wire (014 centim. in 

 diameter), to which the suspending thread is attached, passes 

 through a hole drilled near the upper edge of the disk and is 

 fixed by solder. The copper wire is bent in such a manner 

 as to make the nickel tongue hang horizontally. The copper 

 disk and the nickel tongue, except the extreme point of the 

 latter, are coated with a mixture of lampblack and o-old size 

 to facilitate cooling by radiation. The length of the pendu- 

 lum from the suspending beam to the centre of the bob is 

 85 centim. In order to prevent accidents by burning, the 

 lower part of the suspension for a length of about 10 centim. 

 is made of fine wire ; the remainder consists of ordinary 

 sewing-silk. The round bar-magnet is about 13 centim. long 

 and 1*2 centim. in diameter ; it slides stiffly through a hori- 

 zontal hole in an upright post, and its best position must be 

 found by trial. A sheet of white cardboard is fixed behind 

 the bob, so that the effect may be well s en at a distance. 



The above dimensions might no doubt be varied con- 

 siderably without detriment ; but if they are followed good 

 results may be certainly obtained without waste of time in 

 experiment. 



XVIII. Notices respecting New Books. 



An Elementary Treatise on Hydrodynamics and Sound. By A. B. 

 Basset, M.A., F.R.S., Trinity College, Cambridge. Cambridge : 

 Deighton, Bell, and Co. London: George Bell and Sons. 1890. 



HPHIS is a "middle" treatise on the mathematical theory of 

 -*- Liquid-Dynamics, forming Part I., and Aero-Dynamics, to 

 which Part II. is devoted, as far as needful for the elementary 

 treatment of Sound. It is chiefly designed for students reading 

 for Part I. of the Cambridge Tripos, or other Examinations, in 

 which an elementary knowledge of its subjects is required ; "but " 

 the Author also trusts (to alter slightly his phraseology) " that it 

 will not only be of service to those who have neither the time nor 

 the incliuation to become conversant with the intricacies of the 

 higher mathematics, but will also prepare the way" for those 

 intending to proceed to the higher parts of these subjects. 



Some years back Lord Eayleigh, while filling the post of Caven- 

 dish Professor, threw out a suggestion that the instruction in 

 Pure Mathematics of students who proposed to devote themselves 

 mainly to Physics should be limited, as far as possible, to those 

 parts which are indispensable to this object — a remark particularly 

 applicable to the training of such a " middle " class of students as 

 those for whom chiefly Mr. Basset writes, his present treatise 

 requiring no more than " a knowledge of the elements of Differen- 



