is seen in Fig. 4, where a front view of the limb is given; and here also the coincidence of the two protuberances A, B, as the point of mutual contact, C, is indicated.

In all the figures, the position of the suture is marked;, and, in Figs. 2 and 3, an attempt is also made to shew the positions of the flexor and extensor muscles of this phalanx. These, passing through the last, or the sixth, phalanx, are inserted in the body of the shell below, and beneath the suture of the fifth phalanx above, and it is by them that the action of separation is effected. But the mode of action will be more easily understood, by simplifying the appearance of the parts; as is attempted in that which is rather a diagram than a drawing, at Fig. 9, Plate II. In this, the shell is supposed transparent, to shew the directions and insertions of the separator muscles.

The limb being firmly extended, the protuberance A is thus brought to rest firmly on B at the point C, and in such a manner, that the lower portion of the suture itself just touches the sixth phalanx. If the whole of the fifth phalanx, above the suture, could be thus supported on the sixth, it is evident that the contraction of the flexor and extensor muscles would draw the lower portion of the former towards the shell or body, the latter phalanx being at the same time pressed against it, and thus tend to separate the lower from the upper part of the fifth phalanx. But the necessary freedom of the motion of flexion would not admit of such a construction; and the effect is therefore produced in a different manner.

For this purpose, the chief operation of the extensor muscle is, as before remarked, to fix the protuberance A firmly on B, which thus becomes a fulcrum, or point, on which the upper portion tends to revolve when the flexor is brought into action. Thus the flexor muscle acquires a lever of considerable power; and, on being caused to contract, it draws the lower portion of the fifth phalanx from the upper, and effects the separation. That this is the efficient cause, is rendered evident by the crack commencing at D; although the whole is completed so instantaneously, that it requires a quick eye to perceive a difference between

the commencement and the termination of the process. It is also easy to see that a collateral provision is made for this purpose; as the suture is not only thinner but more feeble at this part; being easily separated by insinuating a knife into it there, while it is scarcely possible to enter the point or edge at the opposite side.

Those of your readers who may find a difficulty in comprehending the nature of this process, from the preceding description, may easily satisfy themselves respecting it, by examining the structure of the parts, in an animal so common. This may be done, even after boiling; when the structure and disposition of the muscles are, indeed, even more easily understood than in the living animal. But I may, I believe, add, that your London readers at least need not be surprised, if their trials of the powers of the living animals, in this respect, should fail; as they are generally too much exhausted in that market, to enable them to display this extraordinary faculty.

EXPLANATION OF THE PLATES I. AND II.

Plate I. Fig. 1, is a whole leg of the spider crab, intended to convey a general view of the articulations and motions of the legs of this tribe. The extreme, or first phalanx, has a direct motion forwards, but cannot be thrown backwards even into a straight line with the second, being checked by the form of the articulation. These motions are effected by two / muscles which occupy the entire length of the second phalanx.

The second phalanx has no direct motions at all, but its lateral motions on each side are tolerably extensive, and are performed by two muscles similarly occupying the cavity of the third phalanx.

The motion of the third phalanx on the fourth is direct; but like the first articulation, this third one is so constructed, that the limb can scarcely be thrown back, even into a straight line. But the construction of the joint allows a very extensive motion forwards, and the connecting ligament is therefore lax and broad. The muscles of flexion and extension occupy the whole cavity of the fourth phalanx.

The fourth articulation is very peculiar, the ligament being so narrow as to be scarcely visible; and, in consequence of the form of the shell and the shortness of the muscles, the motion is very confined. It is lateral

in either direction. The muscles, by which it is effected, are broad, but they arise from above the suture, or white ring, in the fifth phalanx, and are consequently very short.

The fifth articulation, lying between the fifth and sixth phalanges, is very free, and admits of direct motion both ways. But it differs from all the preceding in the great extent of the motion backwards; by which the whole limb admits of being thrown back, so as to be parallel to the flat surface of the animal. It has been already seen that this is the motion which prepares the limb for being detached, and the means by which this is effected will be more fully shown in the following figures. The muscles by which the flexion and extension are performed, and which also by their action serve to separate the limb, are not, as in the former cases, inserted in the next, or sixth, phalanx, but pass quite through it, to be fixed in the shell of the thorax. Their origin is below the white ring, or suture, as that of the last pair was above it.

The sixth and last phalanx is articulated to the body by a very wide and lax ligament, admitting, however, only of a lateral motion, which is performed by muscles inserted in its sides, and fixed at the other extremity to the shell of the body.

Plate I. Fig. 2, represents the sixth and fifth phalanges, with part of the fourth, in a limb of the Cancer Pagurus. The place of the suture, in the fifth phalanx, is less marked in this species; and it was for that reason that the spider crab was preferred, for the purpose of showing that part. It is, however, indicated in the figure. The limb is here thrown back by the action of the extensor; and both the muscles are introduced, as sepa❤ ated from the body and passing through the sixth phalanx. The protberance of the fifth phalanx, which, in the act of detaching the limb, is brought to rest on the sixth at B, is shown at A.

Plate I. Fig. 3, in this figure another view of the same parts is given, for the purpose of shewing the complete extension of the limb at the moment when it is to be detached. At the point C, the protuberance of the fifth phalanx at A bears strongly on the point B in the sixth, or A and B coincide at C.

Plate I. Fig. 4, is a direct view of the interior of the limb at the same place, when in a state of extension. This figure is given principally to show the suture, which is very visible in the interior of the fifth phalanx ; while it also represents the bearing of the fifth and sixth on each other, on the opposite side at C.

Plate II. Fig. 5. The same parts are here represented in a state of moderate flexion, for the purpose of shewing more distinctly the protu

berances of the fifth and sixth phalanges at A and B, which, in the act of extension, are to be brought into contact, or to coincide in a common line C.

Plate II. Fig. 6, is the fifth phalanx, separated from all its connexions; with its flexor and extensor muscles attached at one end, and the lateral muscles, by which it is connected with the fourth phalanx, shewn at the other. The suture is here distinctly seen. This is a joint

of the spider crab.

Plate II. Fig. 7, is a section of the same phalanx, for the purpose of shewing the suture distinctly, together with the insertions of the upper and under muscles. The ends of both those which lie between the fourth and fifth phalanx are shewn; but in consequence of that, only the flexor beneath is visible. It is here distinctly seen, that the shell is reinforced by a ring on each side of the suture; and that it is not only thinner, but distinctly marked by a line indicating the future division between these.

Plate II. Fig. 8, represents the dispositions of the fibres at the suture, after the division has taken place. They radiate from a common

centre.

Plate II. Fig. 9, is a kind of diagram, explanatory of the action by which the separation of the fifth phalanx at the suture is effected. The flexor and extensor muscles which produce the disunion of that part, are represented as if the phalanges were transparent, so that their whole course is seen. The support or resistance which the sixth phalanx gives the fifth, by means of the meeting of the protuberances A and B at C, is also represented. When the limb is thrown into this position of extension by the action of the lower muscles, it is plain that the point C becomes a fulcrum, round which D may be caused to revolve. By means of the action of the extensor, the fifth phalanx is firmly fixed on the sixth, at a point above the suture, and somewhat beneath the upper insertion of that muscle. Thus its principal action becomes that of preserving this posi tion; as it has little or no tendency, from the shortness of the lever thus remaining, to bring back the limb into the state of flexion. Hence that action of the flexor, which would otherwise be exerted in retaining the bent position of the limb, is so counteracted that it tends to draw the phalanx asunder at the suture D, where provision is made for that purpose. It is plain that the action of the extensor tends also to produce the same effect; the great resistance to that flexion, which would other wise defeat this object, being that of the shoulders of the two phalanges at C.

However obscure this subject may at first appear to an observer, from

the obvious tendency of the muscles to bend the limb instead of breaking it, a careful consideration of these drawings, or of the limb of the animal itself with their assistance, will, it is hoped, render it perfectly intelligible.

ART. II.—On the Fogs of the Polar Seas. By George Harvey, Esq., F.R.S., L. and E.

[Communicated by the Author.]

IT has been commonly supposed, that the fogs which cover the Arctic Seas during the greater part of the summer months, are produced by the moist air depositing its vapour, in consequence of being chilled by contact with the sea. But this cause, it is presumed, is not adequate to the formation of mists; since it has been proved by Dr. Wells, that dew and hoar frost are the only results which arise from air, either perfectly or imperfectly saturated with moisture, coming in contact with a body colder than itself. To produce mist or fog, as has been satisfactorily demonstrated by Dr. James Hutton †, it is necessary, that volumes of air, of unequal degrees of temperature, and holding moisture in solution, should be mingled together; and the circumstances of the Arctic Seas, during the period when these fogs generally prevail, are, it is presumed, in perfect accordance with these conditions.

Before the end of June, the shoals of ice are commonly divided and scattered; the temperature of the ocean being at that time necessarily greater than that of the icy masses floating on its bosom. This inequality of temperature will necessarily impart a corresponding influence to the air, and occasion the portions of the atmosphere, resting on the broken surfaces of the water, to become warmer than the atmosphere in the vicinity of the icebergs. The cooling influence of the icy masses also, in consequence of their being elevated considerably above the sea, will be

* WELLS on Dew.

+ Transactions of the Royal Society of Edinburgh, Vol. I,