NATURAL PHILOSOPHY. 149 



Hence, on swinging the ship at Liverpool, immediately on her return to 

 port, the adjusted compasses were found not merely to have retained their 

 southern errors, but that the maximum of the steering compass had increased 

 up to about two points ; whilst correspondently, in a theoretic view, the 

 standard compass had become still more correct than when at Melbourne, its 

 errors on a large number of points being now trifling and unimportant. 



The most important fact remains to be noticed viz. that such was the 

 general accuracy of the compass aloft in this case of the Royal Charter, and 

 such the instruction derived from the repeated swinging of the ship, that the 

 course actually made good on every occasion during the voyage was accu- 

 rately known that is, far within the limits of the defects of steering, and that 

 numerous determinations by azimuths and amplitudes of the variation of the 

 compass in the Pacific and North and South Atlantic, were obtained with 

 perhaps as much accuracy as had ever been had even in ships built of 

 timber. 



COEEECTION OF COMPASSES. 



A late Glasgow invention seems to promise something of importance in ena- 

 bling the compass to work with certainty on iron vessels, although it is difficult 

 to understand the principle. It consists solely in covering the ordinary box 

 with cork or the pith of elder wood, with several coats of resinous varnish or 

 sealing was. The bowl in which the compass is suspended is also lined with 

 the same, and the result is not a complete annihilation, but a very great dimi- 

 nution of the effect of all local attractions. 



OX THE HEAT PEODUCED BY THE INFLUENCE OF THE MAGNET 

 ON BOILEES IN MOTION, BY M. FOUCAULT. 



In 1824, M. Arago observed the remarkable fact of the attraction of the 

 magnetic needle by conducting bodies in motion. The phenomenon appeared 

 singular ; it remained unaccounted for until Faraday announced the important 

 discovery of induced currents. From that time it was proved that, in Arago's 

 experiment, motion gave rise to currents, which, reacting on the magnet, 

 tended to associate with it the mobile body and to attract it in the same 

 direction. It may be said, generally, that the magnet and the conducting 

 body tend, by mutual influence, towards relative repose. 



If, notwithstanding this influence, it be desired that the motion should con- 

 tinue, a certain amount of labor must be bestowed upon it, the movable part 

 seems to be restrained, and this work necessarily produces a dynamic effect, 

 which I have thought, according to the new doctrines, must be attributed to 

 heat. 



TTe arrive at the same conclusion by observing the induced currents which 

 follow in the interior of the body in motion ; but this mode of considering 

 things could give only, with great trouble, an idea of the quantity of heat 

 produced, whilst by considering this character as due to a transformation of 

 work, it appeared to me certain that we should easily produce, in a decisive 

 experiment, a considerable elevation of temperature. 



