[279] 



NOTE ON THE CAUSE OF TIDES. 



BT B. J. CHAPMAN, Ph. D. 



i-ROFESSOa OF MINEBALOGY AND GEOLOGY IN UNIVERSITY COLLEGE, TOBONTO, 



(Condensed from a Communication made to the Canadian Institute, February 7, ISTU.] 



The phenomenon of the tides, stated broadly, consists of a passing 

 elevation, real or apparent, of oceanic -waters at two opposite points 

 on the surface of the globe. These elevations, which follow the moon 

 in its course, may become greatly intensified under local conditions, 

 as where opjiosing coast-lines impede the progress of the tidal wave ; 

 but in the open ocean, it is well known, they are of but slight signifi- 

 cance. According to the received theory, they are occasioned essen- 

 tially by the unequal degree of attraction exerted by the moon on 

 different parts of the earth — this attraction being, of course, modified 

 by that of the sun. It is thus assum.ed that the waters, owing to 

 their comparative mobility, are drawn towards the moon on one side 

 of the globe, whilst the solid earth is drawn away from the waters on 

 the other side — or, to use the common phraseology, is drawn towards 

 the moon faster than the waters can follow. 



This view, although not without opponents, has been almost uni- 

 versally adopted in defaiilt of a more satisfactory explanation. 



The explanation of the cause of tides now suggested has at least 

 this merit : it applies the same principle in elucidation of both tides 

 — that nearest the moon, and that on the opposite side of the globe. 

 It is briefly this : — When two bodies pull against each other, there 

 must necessarily be a contraction of particles towards the centre of 

 each body along the line of pull or resistance. In the pull, therefore, 

 of the earth upon the moon, the earth (and of course the moon also) 

 must suffer a passing contraction : the part along the line of pull, so 

 to say, contracting more than the otjier parts. But this contraction 

 is mechanical only, and is therefore a compression ; and as water is 

 practically incompressible, the sea remains essentially unaffected, 

 whilst the earth shrinks beneath it, and thus causes the tide. The 



