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observations went on in perfect harmony with the understood 

 conditions of the earth, till they came to the neighbourhood of the 

 Himalaya range. It was not unnatural to conceive that the 

 Himalaya mountains would exercise an influence on their levels ; 

 and they did so, attracting the plumb line from the direction it 

 would otherwise have taken towards the centre of the earth. The 

 magnitude of this attraction was measured with much care, and 

 then the theoretical calculation was taken up by Archdeacon Pratt. 

 (When Mr. Pratt was a young man I examined him at Cambridge, 

 and I was then struck by the practical tendency of his knowledge 

 of mathematics.) He examined carefully the measurements of the 

 mountains, and what their disturbance of the plumb-line ought to 

 have been. To his great astonishment he found it ought to have 

 been a great deal more than it really was. It seemed there was 

 something that almost neutralised the attraction of the Himalaya 

 mountains. With the ideas I have on the circumstances, I have 

 no difficulty in suggesting the cause. In the sectional diagram I 

 have here, suppose the black part to represent the mountains, and 

 suppose this lower part to represent lava — as we will call it for 

 want of a better name — and suppose the lava to be much more 

 dense than the mountains, and that the mountains are floating upon 

 that lava. Now, when you see a thing floating upon water or 

 anything else, you are quite sure that that which stands highest 

 above sinks deepest below. If you see a great log of Memel timber, 

 and a small spar or English pole, and the great log rises highest 

 above the water, you are sure that it also goes deeper. So does a 

 great ship go deeper than a boat. Supposing that to be the 

 representation of things here, this follows, — that a great deal of 

 this light matter has displaced a great deal of the heavy lava 

 matter, and the attraction of the upper part is neutralised in great 

 measure by the circumstance that the heavy lava is displaced. 

 That very curious observation seems to reconcile the whole. So 

 far I have said all I have to say respecting the measures. 



The second part of my lecture is founded on what we know 

 about the temperatures. I must mention in the first instance that 

 we know something of the rate at which temperature travels through 



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