for the Advanvement of Science, 95 



ticular branch of this subject, the motion of' the tide- wave in all 

 parts of the ocean, was in such a condition, that by collecting 

 and arranging our existing materials, we should probably be 

 enabled to procure abundant and valuable additions to them. 

 This, therefore, I attempted to do ; and I have embodied the 

 result of this attempt in an " Essay towards a First Approxima- 

 tion to a Map of Cotidal Lines,*" which is now just printed 'm, 

 the Transactions of the Royal Society of London. If the time 

 of the Meeting allows, I would willingly place before you the 

 views at which we have now arrived, and the direction of our 

 labours which these suggest. 



In the case of the science of tides, we have no doubt about 

 the general theory to which the phenomena are to be referred, 

 the law of universal gravitation ; though we still desiderate a 

 clear application of the theory of the details. 



Lighi. — In another subject whichxomes under our review, the 

 science of Light, the prominent point of interest is the selection 

 of the general theory. Sir David Brewster, the author of our 

 Report on this subject, has spoken of " the two rival theories of 

 light," which are, as you are aware, that which makes light to 

 consist in material particles emitted by a luminous body, and that 

 which makes it to consist in undulations propagated through a 

 stationary ether. The rivalry of these theories, so far as they 

 can now be said to be rivals, has been by no means barren of in- 

 terest and instruction during the year which is just elapsed. The 

 discussions on the undulatory theory in our scientific journals 

 have been animated, and cannot, I think, be considered as having 

 left the subject where they found it. The claims of the undula- 

 tory theory, it will be recollected, do not depend only on its ex- 

 plaining the facts which it was originally intended to explain ; 

 but on this, — that the suppositions adopted in order to account 

 for one set of facts, fall in most wonderfully with the supposi- 

 tions requisite to explain a class of facts entirely different ; in 

 the same manner as in the doctrine of gravitation, the law of 

 force which is derived from the revolutions of the planets in 

 their orbits, accounts for the apparently remote facts vf the pre- 

 cession of the equinoxes and the tides. To all this there is no- 

 thing corresponding in theliistory of the theory of emission ; and 

 no one, I think, well acquainted with the subject, would now 



