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N. B. As my house, or rather the site of my barometer, is situated 105.9 feet above the sea level, it must be necessary to add 0-104 inch to all the barometrical heights in the foregoing tables for the correct heights of mercury. The tables contain the means of three daily observations; viz. 8 p.m.; 1 a.m.;

1 and 10 a.m. The most prevailing wind of the 24 hours is only given; and the register thermometer placed in the sun is insulated six feet from any thing capable of reflecting heat, and the scale of it is marked on the glass tube itself. *Helston, Jan. 4, 1823.

M. P. MOYLE.

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Essays on the Construction of Sea Harbours.

By Mr.J. B. Longmire.
(To the Editor of the Annals of Philosophy.)
SIR,

Whitehaven, Jan. 9, 1823. This subject being new to the public, you may deem the following essays worthy of insertion in the Annals, more espe cially as you occasionally have papers in it on civil engineering.

The matter under consideration naturally falls under two parts ; namely, entrances into sea harbours; and the situations and relative positions of the piers of such harbours.

1. Of Entrances. All'entrances into sea harbours may be classed under two heads : in the first, they face the sea, and admit the surf;* and in the second, they face the calmest quarter, while the surf passes to the lee shore without entering the harbour.

In constructing a harbour to make a proper entrance is a very important task; as a harbour that is safe within loses much of its value, if not accessible in every wind that a ship can approach it from the main ocean.

Before describing the theory of entrances, it is necessary to show how vessels approach a harbour in different winds. A ship can sail best with the wind, and her course to any object to the leeward is in a straight line; and although she cannot sail directly against the wind, she can either reach or pass any object to the windward, by making alternate approaches, which have angles of 6_points, or 73° 7' 30", with the wind this is called tacking. When a vessel is sailing with the wind from the

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* The surf is a term applied to express the state of the sea's surface near the shore in a high wind, or in a gale; and signifies the rapid succession of great waves that pass to and strike the shore.

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north point, having her bowsprit N, 73° 7' 30'' W, she is on the
starboard tack; and on the larboard tack, if in the same wind,
her bowsprit point N, 73° 7' 30" E. Thus if a vessel sail along
the linece, fig. 1, toa, the wind
blowing in the direction ga,
she is on the starboard tack,

Fig. 1. the line d e making an angle

D

6 of 73° 7' 30" with the linega; and for the same reasons,

in the same wind, she is on the larboard tack, when sailing along the line ba. Hence by

Hence by a sailing with the wind, or by

g tacking, a vessel can pass from any one place to any other; and of course in one way or the cther she approaches a harbour.

As that entrance which presents itself to the open sea, and admits the surf, can be passed by vessels in all winds; and easier in any wind than the other kind of entrance, I will first treat of it. Let B, fig. 1, be this entrance into the harbour E; D the open sea; A C two piers forming the exterior wall of the harbour; hi a line passing through the middle of the piers; and gaf another line at right angles to them, and in the middle of the entrance meeting with the line' h i. Now the wind blowing in the direction g d is the most adverse to a ship approaching the entrance B from the open sea ; and, as before shown, ba and ca are the lines of approach for this wind. These lines b a and ca make an angle of 13 points, or 146° 15', with each other; and, being the larboard and starboard tacks for the most contrary wind, they have within them all the necessary lines of approach for winds from all points of the compass ; and the harbour E having in front of its entrance 13 points, or 146° 15' of clear

. ' sea room, is accessible in any wind whatever,

The directly contrary wind, and winds within a point on each side of it, would be in gales the worst of all to enter with; but, that coming from the adjoining land, they have not space to raise a high sea near the harbour ; otherwise a vessel in attempting to enter against a gale out of the harbour-mouth, would, to a certainty, be driven out to sea again.

Vessels passing through entrances of this kind can in all winds take shelter within the pier-heads without any assistance from harbour boats to draw them into the entrance, and in gales from

any quarter are not, as in entrances of the other kind, in danger of being driven on the lee shore, provided they are in a good sailing condition and come to the proper place to make the entering tack in side winds. But if vessels arriving in distress by violent storms, by negligence, or want of local knowledge of

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the commanders, miss the harbour altogether, the loss in consequence is not chargeable upon the entrance.

Entrances that face the calmest quarter, and that do not receive the surf, are at right angles, or nearly so, to the main shore.

Tig, a A, fig. 2, be such an entrance to the harbour B,

3 facing the high shore o g; ab is the main pier, and n m the inner

pier.

The exterior lines of approach,

,
eb, cb, are obtained by
allowing such space in
front of the shore at, ik,
and g h, as will keep ships
sailing on these lines from rocks and shallow water; and if they
have within them 146° 15', then a vessel can reach the pier-
head b in any wind. But as the side c is open in strong sea
gales, vessels are some times carried too far past the head b to
turn into the harbour-mouth A ; and the same happens in gales
out of the harbour-mouth A: vessels so driven aside are forced
upon the contiguous rocks or sandy shore, and are destroyed or
much injured. So it has happened after an entrance of this
kind has been tried to a large harbour, and where money could
-be obtained, that outworks have been erected on the exposed
side c, which assimilate the principle of this entrance, when so
modified, very closely with that of the first kind of entrance.

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Sir, yours, &c.

J.B.LONGMIRE.

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On the Geology of Devon and Cornwall.

By the Rev. J.J. Conybeare, MGS.
(To the Editor of the Annals of Philosophy.)
MY DEAR SIR,

Bath Easton, Feb, 2, 1823. The notice of such geological travellers as first visited Cornwall and Devonshire was of course most strongly attracted by those which may be termed their metalliferous districts, and these still do and must always continue to present the most immediate and interesting objects of such research. Soon also the attention was directed to such points of the coast as are

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distinguished either by the magnificence of their scenery, or the extent of the sections and singularity of the phenomena which they exhibit. Many and valuable as the contributions to the geological history of both counties have unquestionably been, yet our knowledge of their structure (especially in the case of such districts as do not fall under the above-mentioned heads) is by no means so perfect as to preclude even in a casual visitor the hope of adding somewhat of information to the present stock. It

may be useful too to point out the deficiencies which yet remain to be supplied by those who have better opportunities and leisure. Such is the object of the following memoranda collected chiefly during the summers of 1809 and 1812, and partly verified in that of 1819. On many points they are of necessity very imperfect, and some parts of the original manuscript have been omitted in consequence of the same phenomena having been far more accurately and fully described in the essay contributed by Mr. Sedgwick to the first part of the Cambridge Philosophical Transactions. What is retained, I have arranged for convenience' sake geologically, rather than topographically; and in this arrangement I venture to propose the following division of the principal rock masses as one which, if not strictly scientific, will yet, I think, be found useful for the purpose.

1. Granite, including some porphyritic beds and mineral veins, and shorl rock.

2. Metalliferous, or, more strictly, cupriferous and stanniferous slate, including various porphyritic and felspathic rocks (elvans), and occasionally greenstone. This I will venture for brevity's sake to term the inferior slate.

3. Slate (which I shall venture for the same reason to term superior), containing no elvans, but abounding much more in greenstone, especially in its obscurer varieties, and in dark coloured limestones. Sparingly metalliferous, containing no tin, and more productive of lead than the inferior. Contains occasionally organic remains.

4. Stratified rock, exhibiting the general character of a conglomerate or sandstone, alternating with tender slate, and occasionally associated with coralline or shelly limestone. Contains no metallic veins, and few if any rocks of the greenstone species. This rock might, perhaps, be regarded as forming the upper portion of No. 3, and both would probably by most geologists be termed greywacke. As any attempt at restricting that term might produce confusion, I shall venture for the present to term this (No. 4) sand slate.* * It is, perhaps, almost needless to remind those who are practised in geological research, that the division of slates here adopted is purely arbitrary, and, for convenience, as the inferior do in fact pass by so imperceptible a gradation into the superior as to render it impossible to assign any determinate limits to each variety, however an examination of points distant inter se may satisfy us of their characteristic difference. It appears to be pretty generally agreed that such a gradation is observable in most of our schistose

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Rev. J.J. Conybeare on the [MARCH 1. Granite. The character of this rock is, so far as I am acquainted with it, remarkably uniform through the whole extent of its range. That of Waterloo Bridge may serve as a specimen. Near the points of junction with the incumbent slates, it occasionally becomes small grained, and of a redder hue, resembling the granite of veins. Examples may be found at a junction near Ivy Bridge, at Bucklund on the Moor, above Belstone (near Okehampton), and near Bovey Tracey at the spot which produced the fine specimens of tourmaline and apatite. As noticed by Mr. Sedgwick in these cases, there appears to be a diminution, sometimes a total loss, of the mica. The predominant variety of granite contains also in many places patches of a smaller grain, generally of a form more or less spherical. These differ so much in their aspect from the general mass, that, by a casual observer,

а. they might be taken for imbedded portions of another rock; a more accurate inspection will soon show that this is not the case, even in those instances in which (as near St. Just) the predominance of dark coloured mica or chlorite in these patches gives them much the aspect of a kind of gneiss. Other instances may be found near the Land's End, near Moreton Hampstead, above Henoch, and on the road leading from Bovey Tracey to the depot of tourmalines, &c. already mentioned.t

The disintegration of granite in situ, as exhibited on a large scale at the porcelain clay pits in St. Stephen's, and the open mine of Carglaise, I near St. Austel, has been often noticed. Another large tract of the same character will be found on Dartmoor, in the neighbourhood of a hamlet termed (from the nature of the soil thus produced) Sandy Park. Other partial instances occur, and the granitic tors, the formation of which has been so ably illustrated by Dr. Macculloch, afford abundant proof that the action of the causes which produced this phenomenon has been at some time or other nearly universal. It has, I believe,

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* See Mr. Sedgwick's paper (p. 10). It may here be noticed that the constituent minerals of granite are seldom found in the west (as in many similar tracts) distinctly or separately crystallized. The imbedded crystals of felspar have been noticed by Mr. S. and others. I found it crystallized in rhombs in small cavities on St. Michael's Mount, and in small rhombs and in larger crystals of a more complicated form at the tourmaline pit near Bovey Tracey. I have also from Cornwall, but without the exact locality, two specimens of perfectly crystallized mica, the one a rhomboidal, the other an hexagonal tablet. Both are in a small grained reddish granite (possibly an elvan), the aspect of which does not resemble that of the specimens brought from Scilly by Mr. Majendic. I may add, that on St. Michael's Mount, I observed in a highly felspathic portion of the granite insulated crystals of felspar rendered as tender as the softest clay by some process of decomposition which had not affected the imbedding mass.

+ Mr. Sedgwick mentions the same phenomenon as observable near Castle Trereen (the Logging Rock): to his remarks I would refer the student for much fuller information.

It is, perhaps, hardly worth while to state, that the finest porcelain clay which I obtained in Cornwall was from a vein in Carglaise. It occurred in very small quantities.

Mr. Taylor (Report, p. 1) affords an interesting illustration of ore result of this process.

“ If the ground, 80 fathoms south-east of Carn-brae, and at Wheal Druid, had not been penetrated by the mines, it is very possible that the whole of it extending

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