known that the waves set up by the model bear the same relation to the size of the model as the waves set up by the ship do to the ship when, and only when, the speed of the model is to the speed of the ship in the ratio of the square root of the ratio of their lengths.

Since this fact has been recognized, most important information has been obtained by experimenting on models. Mr. Froude, by recognizing this law, has been able to bring the comparison of ships by means of their models to such a degree of perfection, that he can now predict with certainty the comparative and actual resistance of ships before they are constructed, and the great practical value of his results have been recognized by the Admiralty. What I propose is virtually to extend these experiments on models so as to make them embrace the steering-powers of ships as well as their resistances. The manner of experimenting would have to be somewhat altered. Steam-launches would have to be substituted for dummy models; but the principle of the experiments would have to remain the same, and the speed of the launches must be regulated by the same law as that of the models.

The turning qualities of such launches might be verified by comparing them with the turning qualities of the ships as found by actual experiment; and then the models might be handed over to the officers of the ships, and they might practice encounters and manœuvres until they knew not only what they could do with their ships, but what it was best to do in order to outmanœuvre each other, and this without any cost or risk.

The behaviour of the models would be in all respects similar to that of the ships, the only difference being that the manoeuvres would be on a smaller scale; and the scale of the manœuvres would be the same as that of the models, so that the step from the models to the large ships would be easy; and familiarity with the working of the ships as well as the models under ordinary circumstances would prepare the officers for using the ships in an actual fight as they have been accustomed to use the models in their friendly encounters. The scheme here proposed has its parallel in military schools. Although "autumn manœuvres and sham fights afford soldiers a much better opportunity of preparing themselves for battle than any thing at present within reach of the sailors, still the war game appears to be growing in favour, and this is nothing more than practising manœuvres in miniature.

Independently of their value as a means of training naval officers, such models would afford a means of studying naval tactics. From them might be learnt the way in which a ship should strive to approach another of nearly equal power and speed, so as to use her ram to the greatest advantage; and of this as yet but very little can be known; and, except on models, it can only be learnt from experiments on the ships.

Important as are the laws which have been verified by the Committee on the steering of screw-steamers, it appears to me that the most important lesson to be learnt from their investigation is, that there is nothing capricious in the behaviour of these ships. To realize the value of this lesson the investigation must be followed up; and it appears that the best way to do this would be by the aid of model launches on the plan thus roughly sketched out.

Seventh Report on Earthquakes in Scotland, drawn up by Dr. BRYCE, F.G.S., F.R.S.E. The Committee consists of Dr. BRYCE, F.G.S., Sir W. THOMSON, F.R.S., J. BROUGH, G. FORBES, F.R.S.E., D. MILNEHOME, F.R.S.E., and P. DRUMMOND.

THE state of quiescence alluded to in last year's Report has suffered scarcely any interruption during the current year. No movement has occurred of sufficient intensity to affect any of the instruments employed by the Committee for testing the shocks. The Association will be aware that these are the seismometer, constructed on the principle of the inverted pendulum, which is placed in the tower of the parish church of Comrie, and two sets of upright cylinders, described in last year's Report, which stand on boards on the sanded floor of a building erected two years ago by the Association upon a site, half a mile west of the Comrie church, kindly granted by P. Drummond, Esq., of Dunearn, in the grounds surrounding his house. This building stands in the Comrie valley, on a boss of rock of the same kind of slate of which the adjacent hills and ridges are composed, and which can be traced into continuity with those on both sides of the valley. It was therefore expected that cylinders so placed would readily respond to any movement affecting the rocks on either side of the valley, more especially as the centre or focus from which it has hitherto been considered that the movements have emanated is at no great distance on the north side of the valley.

This expectation has not been realized, inasmuch as two slight shocks were experienced on the 14th and 16th of January, in the morning and afternoon, without affecting the seismometer or the cylinders, even those of smallest diameter, which a very slight movement is sufficient to lay prostrate in the sand. It is easy to see that a very extreme sensibility must be avoided in order to guard against the effects of other disturbing causes-as a storm of wind, a peal of thunder near at hand, or a heavy footfall on the rock outside; and hence that an undulation, propagated from a distant centre, might be so retarded by the resistance of rocky masses as not to produce the required amount of disturbance. The evidence furnished by several most intelligent and trustworthy persons leaves no doubt that on the day mentioned a very slight shock was really felt on the north side of the valley; that the movement seemed to come from the westward, and was attended by a slight noise, which died gradually away towards the south-east.

This somewhat disappointing result has led your Committee to add two more cylinders of increased delicacy to each set, and to use every effort to obtain suitable sites for other sets more to the west and north, and also further down the valley, as near Dunira, the conjectured focus, and that fixed on by Mr. Milne-Home in the former inquiry, in Glen Lednoch near the edge of the eruptive granite tract, whence the late disturbance seems to have proceeded; and, if possible, also at Ardoch, Dunblane, and Bridge of Allan, at all of which the shocks of 1873 were so severely felt. The expense would be inconsiderable; the difficulty to be encountered is the procuring of a suitable and safe site and a competent observer. Your Committee earnestly hope that these obstacles will be overcome in the course of the succeeding year.

Report on the Present State of our Knowledge of the Crustacea.Part II. On the Homologies of the Dermal Skeleton (continued). By C. SPENCE BATE, F.R.S. &c.

[PLATES II., III.]

As in the first part of this Report the carapace or dorsal surface of the Crustacea was considered, it is now intended to examine the plastron or ventral surface, and so complete our inquiry into the form and structure of the dermal skeleton, previous to a consideration of the internal viscera and development of the animals of the various forms in the class.

The head, or cephalon, is more clearly defined in Edriophthalmous Crustacea than in any other order; but even here the somites posterior to the mandibular ring have the dorsal surface wanting; but a clearly defined character distinctly separates them from the somites that pertain to the succeeding seven, which constitute the pereion.

This condition is less complete in Squilla (which M. Milne-Edwards has selected as being "of all Crustacea that in which the 21 segments of the body are the most distinct"), where the posterior somites of the cephalon as well as the anterior two of the percion are only represented by their ventral surfaces.

This apparent incompleteness of structure, which is due rather to an economy of material, has led carcinologists to consider generally that the cephalon and pereion should be treated anatomically as one portion of the animal under the general name of cephalothorax.

Thus Dana, in writing on the "Classification of Crustacea," in his Report on Crustacea of the United-States Exploring Expedition under Capt. Chas. Wilkes, U.S.N.,' p. 1397, says, "In these highest species, nine segments and nine pairs of appendages out of the fourteen cephalothoracic belong to the senses and mouth, and only five pairs are for locomotion."

This he has taken from the Brachyural or Macrural decapod, as being the highest types of the order; but if we are to report our experiences and define the names and conditions of things according as they are represented in a single type or group, every student of any special form will draw his own conclusions from that which he has alone closely considered, and the study of Crustacea as a class in the animal kingdom must be retarded, if not misrepresented.

In studying scientifically the Crustacea as a whole, it will be found not only more correct but more convenient to describe and name the several parts of the animal by their homologous certainty rather than by their adaptation to fulfil different functions which demand a variation of form with the greater or less importance of their requirements.

The seven somites that form the cephalon are most closely associated, and difficult to be separated from those that follow, in the Brachyural type. This circumstance appears to be largely due to the powerful character of the mandibular appendages. The great strength of these organs requires such an internal development of parts that they appear to preclude the posterior somites from the power of growth; consequently they become merely sufficient to support appendages of a supplementary character.

This is very apparent in the Macrural order. In Palinurus the mandibles are so broad and large that their removal is almost a complete decapitation. It is therefore a structural necessity that the posterior two somites of the cephalon should be supported by those to which they are most closely

approximate; consequently they are frequently found fused with the anterior somites of the pereion.

Yet in this very genus, in a young state, we have the most complete evidence of the limits that define the cephalon from the pereion, and this again from the pleon.

In the larva of Palinurus, as well as in the animal known as Phyllosoma, which is now generally accepted as being the young of Palinurus after some weeks' growth, the cephalon is seen to coincide with the limits of the carapace and terminates anteriorly to the seven somites of the pereion. It therefore appears that it is desirable to identify these first seven somites as belonging to the head or cephalon and that only.

The pereion, or thorax, is also composed of seven somites or segments; and this number is never departed from, even in the most depauperized condition of the animal. These several somites Prof. Milne-Edwards, in his "Observations sur le Squelette tégumentaire des Crustacés décapodes, et sur la Morphologie de ces animaux," Ann. des Sciences Nat. p. 268, 1854, says:— "In order to determine easily each of these anatomical elements of the integumentary skeleton, it is desirable to define them by a name; and I shall call them protosomite, deutosomite, mesosomite, or tritosomite, tetartosomite, pemptosomite, hectosomite, and hebdosomite, following the order which they occupy from before to behind."

In the lower types they form, as in the Amphipoda, separate and distinct segments; but in the higher groups, as we see the dorsal surface of the somites of the cephalon developed and produced posteriorly so as to cover and protect the upper part of the pereion, so we find the somites of this latter division coalesce ventrally more or less perfectly until in the Macrura and Brachyura they reach the highest degree of consolidation and are much more dense and strong than is the structure of the carapace.

This condition is gradually seen to be approached through different stages from the Edriophthalmia upwards. In the genus Squilla (which has many analogies with the sessile-eyed Crustacea, and appears like an enormous stalk-eyed Amphipod) three or four of the posterior somites are exposed beyond the carapace and have the dorsal are complete and separately perfect. In the Diastylidae we see the same; and ultimately in the genus Pagurus, among the Anomurous Crustacea, there is but a single somite that is not embraced within the limits of the carapace, and that is reduced to a very slender ring.

With the deterioration of the dorsal arc of each somite of the pereion the ventral arc increases in density and coalesces the more perfectly with its neighbours. This appears much to depend upon the habits and character of the animal. If it be one whose habits are perambulatory, as in Palinurus, the somites are strongly fused together into a strong broad sternum; whereas in such animals as Palamon and Homarus the sternum is less strongly developed, and apparently of a more feeble character.

This depreciation of the sternum gradually goes on as we approximate the short-tailed orders, and arises from the absorption of the first joint or coxa of the leg into the general system of the animal.

In Palinurus the sternum (Pl. II. fig. 1), corresponding to the posterior five somites, is very broad, and the legs are very widely separated from those on the opposite side; in Homarus, Nephrops, and Astacus (Pl. II. fig. 2) they approximate each other so nearly that the sternum consists of a small calcareous longitudinal cord, to which the apodema are attached and receive their support.