.85,. NOTES AND COMMENTS. 723 



been expended in producing elevations of strata, in inducing molecular 

 changes in rocks, in shearing, &c., still the amount thus expended 

 cannot represent more than a fraction of the whole, for the work of 

 elevating a layer of granite-rock ten miles in thickness for a height 

 of five miles, would represent the heat equivalent to a fall of tempera- 

 ture of only 1° F. throughout the globe. 



Mr. Fisher concludes that there appear to be only two possible 

 replies to the question as to what becomes of the estimated heat generated 

 by this tidal action. Either the earth's crust solidified a very long 

 while subsequent to the genesis of the moon, and so was able to 

 radiate heat directly from its liquid surface into space ; or else the 

 moon was not thrown off at all, but was left behind, according to the 

 nebular hypothesis. In this latter case, the whole amount of tidal 

 action would not have been so great, but still quite sufficient to 

 generate centrically an amount capable of maintaining those internal 

 currents which the theory of a thin crust and a liquid interior 

 appear to necessitate. 



The Thames as a\ Agent in the Dispersal of Plants. 



In the recently issued number of the Linnean Society's Jouvnal 

 ("Botany," vol. xxix., pp. 333 — 346), Mr, H. B. Guppy gives the results 

 of his observations on the agency of the Thames and its tributaries, 

 the Lea and Roding, in the dispersal of plants. Not only in autumn, 

 but during winter and spring, seeds, seed-vessels, and other vegetable 

 drift are constantly carried sea-wards, the months of January and 

 February yielding the richest collections — a fact to be explained by 

 the numerous checks to their progress. A constant wind, blowing 

 at right angles to the river course, will cause a surface-flow across 

 the stream, carrying out of the current much of the drift which is thus 

 often caught in the flags near the side, or embayed for weeks in a 

 sheltered hollow of the bank. In the tortuous channels of the lower 

 Lea, where the current is slow, large collections may be found in the 

 spring in some of the bends, especially after a long spell of east wind. 

 A flood will leave much material stranded until the next one picks it up 

 again. Eddies will detain the drift for days, and, when the tidal part 

 is reached, there is no knowing how many times it may get carried to 

 and fro ; when a seed-vessel arrives off Gravesend, it has probably 

 traversed the distance below Teddington weir many times over. 



Mr. Guppy has worked the Thames from Richmond up to 

 Sunbury weir. By using the tow-net, he says, opposite the weirs^ 

 and collecting the drift in other places where it had become embayed 

 or caught in the flags, he has gathered in a day materials for months 

 of work ; not only seeds and seed-vessels are captured, but bits of 

 stem and branches, from which, if provided with a node, the plant 

 may often be reproduced. Plants of Scutellavia galerictdata and Nastur- 

 tium were raised in this way from small fragments. Shoots of forget- 



3A 2 



