Notices respecting New Books. 405 



periinents are described which prove that the velocity attained 

 when a particular force is acting is directly proportional to the 

 time from the beginning of motion, that for a given mass moved 

 the accelerations are as the forces which produce them, that for 

 a given acceleration the spaces described are as the squares of 

 the times, &c. The student finds that in 2, 3, 4 seconds the 

 spaces are 4, 9, 16 times respectively the space described in the 

 first second. But he is not invited to go beyond these particular 

 cases, nor to find a general expression which shall apply to all his 

 experiments. The formulae s—h gt 2 , v=gt, &c, find no place in 

 this book. Now, much as these and similar formulae are abused 

 by candidates for examinations, who learn them by heart and 

 acquire a rule-of-thumb trick of applying them to problems 

 without having any notion of their physical meaning, we do not 

 think they should have been excluded from a work on elementary 

 mechanics. 



The principles of fluid- pressure are discussed and exemplified at 

 great length ; atmospheric pressure also and the barometer receive 

 a good deal of attention : perhaps, however, a fuller description of 

 the Aneroid Barometer might have been given with advantage. It 

 is an instrument of such common use, that a brief account of the 

 mechanism by which the motion of the lid of the box is transmitted 

 to the index, and the mode of graduating the dial-face, would have 

 been desirable. 



The few pages devoted to the phenomena of air-suction, lateral 

 pressure, Clement's disk and the spray-disperser are very lucid, 

 leaving nothing to be desired in the way of explanation. The 

 author's explanation of the phenomenon of the Clement's disk we 

 will give nearly in his own words : — 



" The effect of suction produced by a current of air is rendered 

 especially obvious if the current is allowed to expand between two 

 flat disks. A circular disk of cardboard 10 centims. in diameter 

 has a hole in the middle. A glass tube about 8 millims. wide, bent 

 at right angles, is passed through a cork which is glued upon the 

 disk so that the bore of the tube is exactly over the hole in the 

 disk. A second disk of stout paper or thin cardboard is suspended 

 to the other by three threads ; the distance between the disks 

 should be 10 millims. If air is strongly blown through the tube 

 it will expand between the plates in a radiating manner, and the 

 particles of air will tend to move with the same velocity. But if 

 the particles of air are to maintain the same velocity, then the 

 same quantity of air which at any instant fills the space within 

 the circle of 1 centim. radius will in the next instant have to 

 fill the space within the ring of 2 centims. radius, in the next 

 that within the ring of 3 centims. radius, and so on. If the par- 

 ticles of air are to maintain their original velocity, it is necessary 

 that the quantity of air which at a certain time fills the inner circle 

 of 1 x 3*14 square centims. area, should fill at the following instant, 

 the ring of (2 2 — 1) x 3*14=3 x3*14, at the next instant the ring 

 of (3 2 — 2 2 ) x 3-14=5 x 3*14 centims. area, and so on j — that is, the 



