176 



NATURE 



[Dec. 26, 1889 



reezing temperature after entering the ground approaches by 

 stages of quick or slow rates, in different parts, towards the sup- 

 posed nearly even depth at last, if we might only presuppose 

 that, because of the endless material obstruction to its motion in 

 any horizontal direction, no channels for the earth's lateral ex- 

 pansion in freezing should subsist ; but that in all places and in 

 all conditions where the freezing happens, the only line of escape 

 of the earth's increase of volume should be vertically upwards 

 towards a direction where no insuperable forces are, at least, 

 opposed to it. 



Were this assumption of upward reliefs only of all of the 

 expansions a really true and valid one, every vertical fibre of 

 the wet earth's mass would behave in freezing quite indepen- 

 dently of every other one, and would take up its fully expanded 

 length at last, no matter at what times and in what order con- 

 gealing overtook its individual portions. A stone, in this sup- 

 position, just embedded in the ground, would have its lower half 

 lifted at last in its socket, and the upper half of the socket 

 lifted off the stone (whether its thermal conductivity is great or 

 small), to the height, in either case, of a water-column's change of 

 length by freezing, whose initial height is but half the vertically 

 measured thickness of the round embedded stone — that is to say, 

 about one-eleventh of an inch for a stone 2 inches in diameter, 

 instead of nearly half an inch, which was about the depth of the 

 settlement, in some of the large-sized flint stones, which was 

 actually observed. 



To return to the reality, however, from this artificial suppo- 

 sition, the actual course of the expansions, and the effects pro- 

 duced by the freezing dilatations must, no doubt, be very 

 different. Supposing that the flint-stones, by their good thermal 

 conductivities, soon become covered with a thickening coat of 

 frozen earth, flow of the soft, unfrozen earth between them will 

 really spring up and be maintained by direct outward expan- 

 sions from the stones of the icy coats surrounding them. On 

 account of the firm rigidity of the exposed earth-surface, to 

 which the stones themselves must soon become fast fixed, the 

 resultant flow of soft earth from between the stones, instead of 

 finding an upward path the easiest, will rather choose a 

 vertically downward one for its escape from its confine- 

 ment, and lift the stones and icy covering together, rather than 

 seek by an upward course to break through the latter. Yet this 

 last effect may also perhaps occur to some extent, raising the 

 frozen earth-caps in some measure off the stones' upper sides, and 

 stretching them, it may be, a little upwards, so as to leave 

 between them and the stones clear empty spaces. That this 

 last effect must be only a secondary and inconspicuous one, 

 however, seems to be pretty obvious from this passingly essayed, 

 and as it now appears all too uselessly pursued and desultory 

 aper^u of the frost's real mode and process of expansive action. 



Regarding the peculiar structures, in fact, altogether from 

 another point of view, and rejecting the imperfect explanation 

 which any one of these presumed congelation processes might 

 at first have been supposed to furnish, of the curiously 

 sunken-lookiug assemblages of the wayside pebbles, an exactly 

 opposite interpretation of their semi-interred condition seems, 

 perhaps, indeed, to afford a more satisfactory and likely explana- 

 tion of it, than the expansive effects of frost in the moist earth 

 were ascertained and shown to have any capabilities and physical 

 resources for. The warmth of the sun, or of wind and rain in 

 some thawing daytime temperature of the generally frosty week, 

 may in short be supposed (which the weather-table of the week, 

 on the 26th and 27th ult. confirms) quite plainly and certainly 

 enough, in consequence of the flints' good thermal conductivities, 

 to have melted and shrunk again to its natural dimensions the 

 hard frozen earth under them, without lowering the level equally 

 of the badly conducting frozen earth surrounding them. Alter- 

 nate days of thaw and nights of frost would, by progressive 

 stages which can be easily traced out and understood, tend quite 

 naturally to exaggerate tliis difference. Thus in another way, 

 but complementary to and at returning times just fitly supple- 

 mented by that first supposed, the problem which the winter 

 scene presented is, still more simply and clearly than before, seen 

 to be solved quite truly and correctly by the relatively high 

 thermal conductivity of the rounded flints as compared with that 

 of the hard frozen earth in which they are enveloped. 



This gradual subsidence, therefore, of flint stones during 

 alternate frosts and thaws, into frozen earth, by consolidation 

 and lateral expansion, followed by liquefaction and vertical con- 

 traction of the water in the earth beneath them, is, it would 

 seem that we may reckon it accordingly, a phenomenon on land 



just analogous and similar to the familiar thermal process which 

 small stones scattered on a smooth frozen glacier-field display 

 in summer-time, by intercepting the heat of the sun's rays, and 

 by sinking to the bottom of the deep water-holes which they thus 

 scoop and delve out for themselves, wherever they happen to 

 have found a lodgment in the naked ice. 



A. S, Herschel. 

 Observatory House, Slough, December 9. 



Foreign Substances attached to Crabs. 



At the last meeting of the Linnean Society I exhibited a 

 number of crabs and certain shells of the genus Phorus having 

 various foreign substances attached to them, about which it is desir- 

 able that more should be known. Some of the crabs manage to 

 fasten bits of sea-weed to the hairs on the carapace and legs ; 

 Polyzoa, Balani, Serpulse, &c., in their earlier stages fasten them- 

 selves on others ; a crab of the Indian Seas — Camposcia retusa 

 — is sometimes completely covered on every part with sand, 

 small shells, and bits of sea-weed — Corallina chiefly. These 

 could only be attached by some adhesive matter, but whence 

 derived ? Droniia vulgaris is occasionally found with a sponge 

 extending over the carapace and almost completely hiding the 

 animal. The species of this genus have the two hinder pairs of 

 legs much reduced, flattened, and lying close to the back, and 

 this is assumed to be an adaptation for the purpose of retaining 

 the sponge. Out of a number of specimens dredged in the Bay 

 of Naples, I recollect only getting one with a sponge on it, and 

 that very soon shrivelled up, leaving a leathery-looking substance 

 attached to the base of the carapace, not held by the legs 

 apparently.^ Two crabs — yEthusa mascarone and Dorippe 

 lanata — having similarly reduced hind-legs, but directed upwards, 

 seem much better adapted for retaining a foreign substance, 

 which, however, they are not known to do. In a Mauritian crab 

 — Dynomene hispida — the hind pair only are reduced, but to 

 such an extent as to be merely rudimentary and incapable of any 

 use. Paramithrax barbutus — a New Zealand crab — has, like some 

 others, hooked hairs, but in the specimen exhibited they appear 

 to be free of any foreign substances, although many small frag- 

 ments of an uncertain nature appear between them. 



In Phorus a strong cement only could hold on those large and 

 heavy substances — shells, stones, &c. — completely covering the 

 shell, as in P. agglutinans. I have not seen any account of 

 their modus operandi, but, as the animals have a long proboscis, it 

 is possible that that may be the organ employed, but it is diffi- 

 cult to believe that it would be able to lift any large substance, 

 or that it could reach the top of the shell. Another difficulty is 

 that they must cast off, from time to time as they grow, the 

 smaller substances, to replace them by larger ones. There is one 

 Phorus, however — P. calyctdatus — in which small shells imbed 

 themselves at short intervals along the whorls, leaving the 

 greater part of the shell uncovered ; these little cup-shaped de- 

 pressions are marked inside, as far as the mouth of the shell will 

 permit them to be seen, by corresponding protuberances. This 

 would seem to indicate a certain softening of the shell at one 

 time or other. 



I do not see where protection comes in, in any of these cases. 



December 14. Francis P. Pascoe. 



A Marine Millipede. 



In the hopes of arousing the interest and the energies of 

 British entomological collectors, "D. W. T.," in a short notice 

 on p. 104 of the present volume of Nature, draws attention to 

 the recent discovery in Jersey, by Mr. Sinel, of that remarkable 

 marine centipede Geophilus (Schendyla) sub?narinus (not sttb- 

 7naritimus by the way), of Grube. 



Those who observed this notice, and are interested in the 

 fauna of Great Britain, may be glad to hear in addition that 

 more than twenty years ago a number of specimens of this then 

 undescribed species were taken by Mr. Laughrin at Polperro on 

 the south coast of Cornwall. These specimens, which were 

 presented to the British Museum in 1868, were found associated 

 with Linotania viaritima (Leach) — also a marine centipede— 



' Bell, in his "British Crustacea" (p. 371), states having received 

 " numerous young specimens from Sicily, every one of which had the cara- 

 pace entirely covered with a sponge, which had grown over it, conceahng 

 even the two hinder pairs of legs, which were closely placed against the back, 

 and rendered immovable." No mention is made of a sponge on those that 

 came from the Channel. 



