'Researches in the Undulatory Theory of Light* 181 
I made the following experiment on the velocity of waves 
out at sea. Lat. 2' S. Long. 27° 25' W. 
There was a s^ell on the sea moving from fore aft ; wind only 
sufficient to carry the vessel (all sails set) steadily two or three 
miles an hour. Two large floats were connected by a line 
forty fathoms in length, the line itself being supported on the 
surface of the water by smaller floats. This apparatus was 
towed astern by a long line connected with one of the large 
floats by one end, the other end being wound round a reel. 
The chief officer watched the chronometer ; the second 
officer held the reel fixed ; and I observed the large floats. 
A few seconds before the first float was raised to its greatest 
height b}^ a given wave, I gave a signal to the second officer 
to let the reel run, and immediately the floats became sta- 
tionary in the water. 
At the instant the first float, and also at the instant the se- 
cond float, was raised to its greatest height by the wave al- 
ready mentioned, I gave audible signals to the chief officer, 
who marked the interval of time between the signals. 
A very good average of many trials gave a trifle less than 
six seconds of time for the motion of the wave from float to 
float, i. e. over forty fathoms. This gives nearly 27^ statute 
miles an hour. 
The chief officer and I changed places, and came to the 
same result. Two days afterwards this was confirmed, 
though in rather a rough manner, by observing the motion 
of the vessel by a swell moving abaft. 
Calcutta. J. H. P. 
XXXIII. Researches in the Undidatory Theory of Light con- 
tinuedi On the Absorption of Light, By John Tovey, 
Esq. 
(Continued from p. 455 of last Volume.) 
To the Editors of the Philosophical Magazine and Journal. 
Gentlemen, 
Y^GU will have observed that the formulae of my last com- 
^ munication are deduced from the fundamental principles 
of the undidatory theory without the aid of any assumption 
respecting the arrangement of the molecules, or the nature of 
the constant quantities, Z", &c., which appear in the integra- 
tion. Those formulae indicate, as was shown, that the trans- 
mission of the light may be accompanied by an absorption, 
or diminution of its intensity, depending on the nature and 
thickness of the medium through which it is transmitted, 
