62 Royal Society. 



moving, as the whole of its track, for a certain distance, appears to be 

 equally illuminated; but by combining- a rapid transverse motion of 

 the body from which the light proceeds, with that which it had before, 

 its path may be lengthened to any assignable extent, and both its 

 duration and its velocity will admit of measurement. The author gives 

 various illustrations of this principle, and of his attempts to apply it 

 to appreciate the duration and the velocity of the electric spark. His 

 first experiments were made by revolving rapidly the electric appa- 

 ratus giving electric sparks ; but in every instance they appeared to 

 be perfectly instantaneous. He next resorted to the more convenient 

 plan of viewing the image of the spark reflected from a plane mirror, 

 which, by means of a train of wheels, was kept in rapid rotation on a 

 horizontal axis. The number of revolutions performed by the mirror was 

 ascertained, by means of the sound of a siren connected with it, and still 

 more successfully by that of an arm striking against a card, to be 800 

 in a second. The angular motion of the image being twice as great as 

 that of the mirror, it was easy to compute the interval of time occupied 

 by the light during its appearance in two successive points of its ap- 

 parent path, when thus viewed ; and it was ascertained that the image 

 passed over half a degree (an angle which, being equal to about an 

 inch, seen at a distance often feet, is easily detected by the eye,) in 

 the 1,1 52,000th part of a second. The result of these experiments, 

 as regarded the duration of the spark, was that it did not occupy even 

 this minute portion of time; but when the electric discharge of a 

 battery was made to pass through a copper wire of half a mile in 

 length, interrupted both in the middle, and also at its two extremities, 

 so as to present three sparks, they each gave a spectrum considerably 

 elongated, and indicating a duration of the spark of the 24,000th part 

 of a second. The sparks at both extremities of the circuit were perfectly 

 simultaneous, both in their period of commencement and termination ; 

 but that which took place in the middle of the circuit, though of equal 

 duration with the former, occurred later, by at least the millionth part 

 of a second, indicating a velocity of transmission from the former point 

 to the latter of nearly 288,000 miles in a second ; a velocity which 

 exceeds that of light itself. 



The following letter was read from the Chair. 



" British Museum, June 19th, 1834. 



" My Dear Sir, — His Royal Highness the President requests 

 that, when you adjourn the meeting this evening to the 20th of No- 

 vember, you will have the goodness to express his great regret that, un- 

 fortunately, the state of his healtli and sight has lately been such as to 

 render it impossible for him to preside at the ordinary meetings of 

 the Society so frequently as it was his anxious wish to have done. 

 His Royal Highness begs you will assure the Society that his absence 

 has been occasioned by the cause alluded to alone, and from no feel- 

 ing of diminished interest in the prosperity of the Royal Society, or of 

 regard and respect for the Fellows; on the contrary. His Royal High- 

 ness hopes that, by the blessing of Providence, his liculth will soon be in 

 all respect"? so far re-established as to enable him, on the reassembling 



