Mechanical Science. 385 



effects be the same ? If in place of making the iron disc revolve 

 and holding a steel file to it, the disc were at rest and the steel 

 file revolving with equal rapidity round it, the same part being 

 always in contact with the disc, is there any reason to expect a 

 different result than that now obtained ? We think not ; and, 

 indeed, going at once to the principle sought to be established, 

 we cannot think that action and reaction are distinguished from 

 each other by any difference in the times which they respectfully 

 require, or that where there is time for the one there is not time 

 for the other. 



9. Magnetic Rotation.— -M. Arago's beautiful experiment is now 

 well known, and, as it deserves, attracts attention every where 

 The following are some results obtained by MM. Prevost and 

 Colladon, which, as they vary slightly in certain points from those 

 as yet published in this country, will be interesting to such as 

 pursue this branch of science. 



A disc formed of a thick copper wire rolled in a spiral, pro- 

 duced much less effect than a perfect disc of the metal of the same 

 weight and size. 



A disc of glass covered with lead, or a single leaf of tin glued 

 on to wood, sensibly deviated the needle. Wood alone, or sul- 

 phur, or a disc of peroxide of iron, had no appreciable effect. 



A disc of hammered copper deviated the needle more strongly 

 than the same disc annealed. 



A screen of copper, or copper and zinc interposed, diminished 

 the effect without destroying it. The diminution was greater as 

 the screen was thicker, or placed nearer to the needle. A screen 

 of glass had no influence. If the interposed metallic screen were 

 pierced by an aperture equal in diameter to the length of the 

 needle, its effect was very nearly the same. 



A vertical magnet suspended in the centre of a cylinder of cop- 

 per remained unmoved, whatever the direction or rapidity of rota- 

 tion of the ring. 



When two needles were fixed together in a similar direction, the 

 effect increased ; when they were placed with their opposite poles 

 together, it ceased entirely. 



A needle magnetized, so as to have similar poles at its two 

 extremities, was the apparatus most sensible to the motion of the 

 discs. It was one of this kind which the authors used in their 

 delicate experiments. 



The conclusion arrived at by MM. Prevost and Colladon is, that 

 the effects are due to a transient magnetization of the discs, which, 

 not being able to modify itself with a rapidity proportional to that 

 by which the different points of the disc are displaced by rotation, 

 are transported to a small angular distance from the needle before 



