Capt. Thomson on the Velocity of Waves. 405 



sition, but differs greatly in specific gravity and combustibility 

 when prepared at different times, even from the same kind of 

 oil, owing to variations of temperature and other circumstances. 

 (Annals of Philos. March 1821.) — The Doctor does not in- 

 form us what temperature is most favourable to its generation. 

 I have alluded to an etherial fluid which appears also to have 

 attracted his attention. Whether they are one and the same, 

 or whether on account of the form of my apparatus the fluid 

 may have undergone any modification in its constitution, at 

 present I am unable to determine. The following is that to 

 which I allude : — Essentially the gases from oil and coal are 

 composed of the same ingredients, though in different propor- 

 tions, viz. simple hydrogen, light carburetted hydrogen, and 

 carbonic oxide gases, with the addition of variable proportions 

 of an elastic fluid, which agrees with olefiant gas in being con- 

 densable by chlorine, but consumes more oxygen and gives more 

 carbonic acid by combustion, and has a higher specific gravity 

 than olefiant gas, and even than atmospheric air. Whether 

 this ingredient be strictly a gas permanent at all temperatures, 

 or a mixture of olefiant gas with some new gas, constituted of 

 hydrogen and charcoal in different proportions from what are 

 found in the known compounds of those elements, or merely 

 the vapour of a volatile oil, he leaves to be decided by a future 

 train of experiments. (Annals, Mar. 1821.) — Though these 

 are not his own words, they contain the substance of his 

 meaning. I am, gentlemen, 



Your most obedient servant, 

 Charles-street, Sheffield, April 26, 1823. JoHN Elliott. 



LXXXIII. On the Velocity of the Waves of the Sea. By 

 Capt. David Thomson. 



To the Editors of the Philosophical Magazifie and Journal. 



THE subject of the following communication relating to the 

 velocity of the waves of the sea, seems to have been very 

 much neglected. My attention was first directed to the method 

 of finding the velocity of the waves by the remarks of Captain 

 Horsburgh in that excellent work The East India Directory. 

 The following is the method recommended by Captain Hors- 

 burg for ascertaining the velocity of the waves : — 



When a ship is sailing in the same direction as the waves 

 are proceeding, — at the time that the log is hove let a person 

 observe the instant that the log is on the top of a wave, and 

 give notice to one holding a watch ; the person observing the 

 wave is again to give notice when it just comes to the stern of 



the 



