Proceedings of the Royal Society ofEdmburgh. 197 



comparative experiments were made in vacuo. He considers, how- 

 ever, that if the needle be prepared, by being previously exposed to 

 a variation of temperature from 212° to 0" of Fahrenheit, its tension 

 will not afterwards be affected by ranges of temperature within these 

 limits- 

 One of the most interesting parts of Mr Harris's paper is his mode 

 of determining changes of magnetic tension in a particular magnet. 

 It is well known that if a needle be made to vibrate within a ring 

 of copper, it will be more speedily brought to rest, than if vibrating 

 in open space. The influence of the ring of copper, therefore, might 

 be employed to detect changes in tension, provided the force which 

 induced motion in the needle, and that force by which it would eventu- 

 ally be reduced to rest without the ring, were both constant quantities. 

 This, however, is not the case ; but the author proposes to reverse 

 the experiment, and cause the ring to vibrate round the needle, placed 

 within it, so as just not to touch the ring. This will afford a com- 

 parative measure of the force of the needle at its poles, if we observe 

 the influence of the needle in reducing the ring to a state of rest. A 

 convenient mode of doing this he has given, and has deduced a ge- 

 neral formula for estimating the differences in magnetic tension thus 

 detected. 



The author has also examined the influence of bright sunshine on 

 the suspended needles ; and has shewn that the difference observed 

 in the oscillations of the needle in sunshine and in the shade, may 

 be made nearly to vanish in the exhausted receiver ; and he has 

 rendered it probable, that the slight differences observed in bars 

 oscillating in the sun's rays, are not altogether dependent on mag- 

 netism. 



Lastly, the author endeavoured, by an artificial electric aurora in 

 a luminous conductor, six feet long and four inches wide, to ascer- 

 tain whether there was any effect produced on a finely suspended 

 needle, placed within eighteen inches of the conductor ; but the os- 

 cillations of the needle were not affected by a stream of electricity 

 procured for twenty minutes from a powerful machine in this appa- 

 ratus, 



2. Experiments on Magnetic Intensity made at Liverpool 

 and Manchester. By Dr Traill. 



Dr Traill made a report to the Society of experiments made by 

 him in 1832 at Liverpool and Manchester on magnetic intensity, 

 measured by the oscillations of the horizontal needles belonging to 

 the Society, which had been sent to him for that purpose. The 

 ' reporter also stated the result of a series of experiments made 

 by Professor Oersted and himself in Liverpool in J 823, which is 

 important, as having been made use of by Professor Hansteen in 

 constructing his isodynnmic magnetic lines for Great Britain. 



The result of Dr Traill's experiments is, that Hansteen has estimated 

 the magnetic intensity of England a little too high, as Mr Dunlop 

 found he had that of Scotland ; and the reporter concluded that this 

 arose from the experiments on which Hansteen founded his calcula- 



