1847.] WHEWELL ON THE WAVE OF TRANSLATION. 229 



vian chain for the source of diffusion instead of a single centre, the 

 distance travelled by each mass will be the same as in the supposed 

 circle, which we may therefore make the basis of calculation. 



Within the circle of 800 miles radius, I will take an inner circle of 

 200 miles radius, and I will consider the drift as occupying only the 

 annular space between these two circles. 



The mean distance from the centre of the annulus lies along a circle 

 of which the radius is 500 miles. I will first consider places at this 

 mean distance. 



I must necessarily make some supposition about the mass of the 

 materials which compose the drift. Let it be supposed that, at this 

 mean distance from the centre of diffusion, every square mile, on an 

 average, contains as much drift as would cover it entirely to the depth 

 of one hundredth of a foot. This is equivalent to supposing that 

 there is on each mile, a patch of drift, one-tenth of a mile square and 

 one foot deep ; or a ridge or " trainee " of drift, one-tenth of a mile 

 long, one-hundredth of a mile broad, and ten feet deep. It is easy 

 to see that the supposition might be put in innumerable other forms ; 

 and by comparing these with many observed facts, some average 

 result might perhaps be obtained. 



Supposing this result to be, as I have said, that on every mile there 

 is an average depth of one hundredth of a foot, I shall, for the sake 

 of easy calculation, call this -^^q of a mile (instead of g^g;^^^). And 

 thus, on every square mile of ground, at the mean distance from the 

 origin, there is 500^000 of ^ cubic mile of drift. 



I will suppose the mean specific gravity of this material to be three 

 times that of water. When the materials are immersed in water, the 

 effective gravity will therefore be twice that of water. 



The horizontal force which it requires to move a body along a 

 surface on which it rests, depends on the form of the body, its texture 

 and that of the surface, and other circumstances : but I think we 

 may suppose that it would require a force and pressure of at least one- 

 fourth the weight of the mass moved, to propel rocks and loose ma- 

 terials along the bottom of the sea. 



This being assumed, it will require a force (pressure) equal to the 

 weight of half a cubic foot of water to move a cubic foot of drift ; 

 and so, for any other quantities. And to move 505^ of a cubic mile 

 of drift, will require the weight of 1^000,000 of ^ i^ile of water, acting as 

 a pressure. 



Now this mass of drift, which is found on an average mile at the 



