294 Mr. Herschel on the Astro7iomical Causes 



which regards them rather as regular and necessary effects of great and ge- 

 neral causes, than as resulting from a series of convulsions and catastrophes 

 regulated by no laws and reducible to no fixed principles, the mind naturally 

 turns to those immense periods with whose existence in the planetary system 

 the astronomer is familiar ; at first attracted by the analogy offered by a dura- 

 tion commensurate to those lapses of time which geology contemplates, and 

 afterwards with a hope of discovering something in the fluctuations to which 

 the orbit of our own planet is liable, which may tender a reason for at least 

 some of the events in its geological history. 



The sun and moon are the only bodies in our system whose influence can 

 at all directly affect the condition of our globe : both by their effect in causing 

 tides — the former by its heat. The tide produced by any luminary is, as is 

 well known, inversely as the cube of its distance. Hence it is evident that 

 any considerable approach of the moon to the earth would greatly increase 

 the tides. If, for instance, the mean distance of the moon were diminished 

 by only one tenth of its actual amount, the mean rise and fall of the tides 

 would be increased by a full third of their present quantity, which would of 

 course produce a great increase in their erosive action on the continents, as 

 well as in the transporting power of the waters of the ocean over the materials 

 of the land. 



The mean distance of the moon is actually on the decrease, and has been 

 so from the earliest ages, producing the astronomical phajnomenon known by 

 the name of the "acceleration of the moon's mean motion." But this decrease, 

 which takes place with extreme slowness, has been demonstrated by Laplace 

 to be incapable of going to any such extent as that above spoken of, and, after 

 a period of enormous length, will be again converted into an increase, which, 

 in like manner, will never be so great as to make any considerable change in 

 the relations now contemplated. 



The excentricity of the lunar orbit is also liable to fluctuation ; and it is far 

 from proved that, if we extend our views backwards for many millions of years, 

 it may not formerly have been materially greater than at present, in conse- 

 quence of some extensive periodical inequality, or the accumulation of many 

 such. Now should this ever have been the case, the tides in the perigee of 

 the moon would of course have experienced a corresponding increase. But 

 there is no reason to believe that any possible approximation of the moon to 

 the earth, arising from increased excentricity in her orbit, sliould have brought 

 her to within two thirds of her actual perigean distance ; in which supposition 

 (purposely assumed as an extravagant one) the lunar tide would still have had 

 less than 3|^ times its present magnitude ; one which no doubt would suffice. 



