Further in

cated by Captain ROBINSON, is about the same distance from the meeting of the tide in the Irish Sea as the North Sea node is from the meeting of the waters off Dungeness, and is similarly situated with respect to the node of Courtown as the North Sea node is with regard to Swanage.

It appears therefore that the tidal feature of these two channels corresponds in almost every particular.

I cannot but consider the identity very remarkable and interesting, and especially so as concerns the relative situations of the nodes of the wave, hinging (as those in vestigation the channel appear to do) upon a single point, and not upon a negative line across phenomena the channel, as may have been imagined. And it seems highly desirable that a

of the tidal

of English Channel urged.

critical investigation of the phenomena of this channel should be made; not only for the purposes of science, but especially on account of the navigation of the strait, for already we may trace the cause of vessels entering the channel, being set down upon the French coast about Heaux; and who can call to mind the lamentable loss of our Indiamen on the coast about Boulogne, and not be reminded that these disasters occurred very near the point where the stream may probably be turned down upon that shore by the meeting of the tides off there?

Having now, Sir, placed before you the result of our observations upon the tides of the Irish Sea, and shown the connection which exists between the phenomena of the Irish and English Channels, and the possibility there is of forming into a system the apparently contradictory directions of the stream, at the mouths of those estuaries, which is so very desirable, I have to request that, should you, after the perusal of this letter, approve of the observations being continued round the Land's End and up the English Channel, you will solicit the Lords Commissioners of the Admiralty to furnish the means of so doing.

I have the honour to be, Sir,

To Rear-Admiral Beaufort,

&c. &c. &c.,

Hydrographer.

Your obedient humble Servant,

F. W. BEECHEY, Captain.

VII. On the Blow-hole of the Porpoise. By FRANCIS SIBSON, Esq.
Communicated by THOMAS BELL, Esq., F.R.S.

Received December 18, 1845,-Read March 12, 1846.

THE Porpoise inspires through the blow-hole which includes the nostrils, and is so placed that the animal can breathe with only a very small portion of its head above the water. A large passage leads down from the external opening to two canals that pass directly downwards through the skull, between the cranial and facial bones; these nasal canals are separated from each other by a thin plate of bone (Plate XII. fig. 1.3-26.27)*. The two canals, after passing down through the skull, coalesce to form a muscular tube that opens at its lower part into the pharynx, by a constricted aperture. Through this aperture the mouth of the larynx rises, after the larynx itself has projected quite through the pharynx, dividing it into two channels.

Connected with the channel that leads from the external opening down to the two bony conduits is a series of pouches; these are composed of aponeurotic walls, folded on themselves in many wrinkles, so as to be capable of great distension.

The two largest and most dilatable of these pouches are seated on each side and in front of the outer passage, into which they open separately (fig. 1.2-1).

Deeper than, and between these two anterior lateral pouches, is the anterior,central pouch, which is placed upon the combined maxillary bones, and communicates with the nasal canal by a large opening guarded by two oblong cartilages; this sac has a muscular coat (fig. 1.2-2).

The openings into the two nasal canals are just behind and below this anterior deep pouch.

Immediately above the apertures of the two bony canals are two other horizontal apertures that communicate with two corrugated dilatable pouches (fig. 1.2-3). These posterior deep sacs are immediately behind the external channel, and are seated between it and the frontal bone; their openings look downwards to the bony canals, their summits rise directly upwards in front of the frontal bones.

The five pouches are under complete muscular control.

When filled, their apertures can be closed, and their contents retained; or the apertures can be thrown open, and their contents pressed out.

The pouches, when distended with air, doubtless act so as to float the external

* Figure 1 must be looked at, not in the direction of the plate, but sideways.

† See Cyclopædia of Anatomy, i. 580, fig. 269, Article Cetacea, by Professor OWEN.

opening uppermost, and retain that opening above the surface of the water during sleep and during the act of copulation.

The muscles that open and close the blow-hole, and that act upon the various sacs, form one of the most complicated yet most exquisitely adjusted pieces of machinery that either nature or art presents.

The blow-hole is stretched open, in front, by two muscles (fig. 2–6) that act from the superior maxillary bones, to draw forward the cartilaginous mass in front of the outer opening; behind, by muscular fibres (fig. 2-7) that act from the frontal bones on the posterior and lateral edges of the blow-hole.

To close the blow-hole, two sets of muscular fibres (fig. 2-5) spring from the frontal bone, pass obliquely forwards and inwards, one to each side of the opening, and are inserted into the gristly mass (fig. 2-4) in front of the blow-hole. When they act, they draw this gristly mass forcibly backwards, and close the blow-hole by pressing its anterior and posterior walls together, and compressing them against the posterior pouches and the prominence at the centre of the frontal bone.

A set of muscular fibres (fig. 2-8) arises from the circuit of a depression formed on the cranium to each side of the bony canals; these fibres pass over the anterior lateral pouch, blending with the muscles that close and open the blow-hole. On removing this, a covering of muscular fibres (fig. 2-8 a) that cling round the sac is exposed; these arise from the anterior lateral part of the superior maxillary bones, and pass upwards and backwards to form a muscular envelope for the pouch on its inner and posterior surface; the fibres return forwards to be inserted into the outer and anterior walls of the blow-hole.

A fan-like muscle (fig. 2-9) arises from the circuit of the depression to be inserted into the posterior and lateral walls of the blow-hole; this assists in opening it by spreading apart its walls.

On removing this fan-like muscle, a remarkable series of similar fan-like muscles (11.12.13.14.15*) is exposed. These lie one above the other, and arise by muscular fibres from the circuit of the depression; the fibres of each converge to a tendinous aponeurosis thinner than paper.

Four or five of these fan-like thin muscles may be removed by careful dissection, one after another; they act to expand different parts of the outlet of the anterior lateral sac, to draw backwards and expand the posterior walls of the great respiratory canal, and to compress the posterior deep sacs.

On removing these, two remarkable muscles (fig. 1. Archives, fig. 7-22.23) are brought into view that have each a double muscular origin, one from behind forwards from the frontal bone (22 a. 23 a*), meeting another from before backwards and inwards from the superior maxillary bone (22 b. 23 b*). These two origins combine in each to form a tendon; these tendons (22.23) pass forwards in front, and below the neck of the anterior pouch, where they unite with their fellows; each of these tendons * These numbers refer to drawings that are in the Archives of the Royal Society.