mentioned my views on the subject to several gentlemen qualified to judge of their correctness, and soon had an opportunity of putting them to the test of experiment. I received an order to make one for Mr Lees, lecturer on mechanical philosophy in the School of Arts here, on condition that he was to be permitted to return it, if, on trial, it was not found capable of executing all that I had taught him to expect. This pump, through the kindness of Mr Lees, in whose possession it has been for the last eighteen months, was exhibited to the Society for the Improvement of the Useful Arts, on 19th December 1827*.

That the peculiarities of the construction of my pump may be more readily perceived, I shall first shortly describe the common construction, and then its most perfect form, as improved by Cuthbertson.

The common air-pump consists of two barrels AA', Fig. 1., Plate IV, in which the pistons PP are fitted and moved by the racks RR and pinion O, the pistons being thus raised and depressed alternately by turning the winch W. In the bottoms of the barrels there are openings, communicating with the receiver or bell-glass; over these openings valves of waxed silk or bladder are so placed as to admit of the passage of the air from the receiver through them, but to oppose its passage from the barrels to the receiver. It is obvious, that, on drawing up either of the pistons, a vacuum will be formed under it till the air from the receiver, by its elastic force, opening the valve V or V', distributes itself equally betwixt the barrel and receiver. Now, as the pistons are furnished with valves PP' of the same kind, and opening in the same direction as VV', on pushing either piston down, the air in the under part of the barrel being prevented by the valve at the bottom from returning to the receiver, will open the piston-valve and escape into the apartment, with the air of which the piston-valve communicates; and these effects will follow the raising and depressing of the pistons, as long as the air in the receiver has sufficient elasticity to open

* The instrument had been previously submitted to the examination of Dr Turner, one of the Secretaries of the Society of Arts, who reported that he had minutely examined it, and was perfectly satisfied with its performance. On his representation to the Council of the London University, I have since received an order to make one for the chemical class of that institution.

2

the valves VV. When it can no longer effect this, the exhaustion muste cease, and, consequently, a near approximation to a vacuum cannot be obtained by means of this construction.gi

The best method hitherto proposed for effecting a more perfect exhaustion, is that of Cuthbertson, which proceeds upon the principle of opening the valves independently of the elasticity of the air; and, accordingly, he substitutes in the place of the bladder-valves VV', Fig. 1., the metallic ones VV' Fig. 2. *, having the wires WW attached to them, which slip stiffly in stuffing-boxes in the piston-rods. On raising either piston, the valve V or V' is opened by the friction of its wire in the stuffingbox, and is shut by its depression; in the former case leaving a free communication betwixt the part of the barrel under the piston, and in the latter case cutting it off. In the pistons he also places metallic valves PP', to be opened by the descent of the piston-rod, and to be shut by its ascent, the valves in the pistons thus opening and continuing open, while those at the bottom of the barrels are shut, and vice versa. Now, as the piston-valves are opening while those at the bottom of the barrels are shutting, Cuthbertson found it necessary (though this is not required in the common pump) to exclude the external air from the barrels.

For this purpose he put air-tight covers CC' over the barrels, and made the piston-rods move in the air-tight stuffing boxes BB', and placed metallic valves MM' in the covers for the egress of the air, to be opened either by its elasticity or by the pistons striking against the projecting points pp' of these valves.

It is also necessary to prevent the return of the air into the pump during the shutting of these valves, which is done by having them immersed in oil.

This construction is certainly, in theory, as near perfection as we are likely to reach by any form of pump, but it is as certainly very complex, and, consequently, very expensive, and liable to go out of repair,-an objection of which those who have been engaged in making these pumps best know the force. Î

Believing the only useful part of Cuthbertson's invention to be the contrivance for opening the valves at the bottoms of the barrels mechanically, I was of opinion a pump would perform near* In figures 2d and 3d only one of the barrels are represented.

ly, or altogether as well, divested of all the other peculiarities of his instrument, and possessing the decided advantages of be ing cheaper and much more easily kept in order.

Fig. 3. is a section of the barrels of my pump, in which I employ metallic valves at the bottom of the barrels, and waxed silk ones S S' in the pistons, laying aside Cuthbertson's metalSin lic valves in the pistons, removing all his apparatus from the top of the barrels, and leaving the pistons exposed to the atmo sphere, as I consider all those contrivances to be unnecessary, although it has been uniformly held essential to a good air-pump, since the time of Smeaton's invention, that the pressure of the atmosphere should be taken off the piston-valves; and my rea son for doing so is, that the air will be always so compressed in the barrels, by the descent of the pistons, as of itself to have sufficient elastic force to open the silk valves in the pistons, the capacity of the barrels being each several thousand times greater than the space betwixt the two valves, when the piston is at the bottom. In fact, by making the under side of the piston and the bottom of the barrel fit each other, which, with the assistance of the oil employed in the barrels may be done perfectly, there will be no space left but the small hole in the piston to its valve.

For illustration, let us suppose the stroke to be 12 inches, and the diameter of the barrels 2 inches, or 25 tenths (as is the case in Mr Lees' one), the diameter of the hole e one-tenth of an inch, and its length 1 inch, their circles being to each other as the squares of their diameters, we have 1x1 = 1 for the capacity of the hole, and 25 x 25 x 12 = 7500 for the capacity of the barrels; and consequently air, which, in the receiver was 7000 times rarer than the atmosphere, would have sufficient elastic force to open the valve in the piston; but as this is a degree of rarefaction far beyond what has ever been attained, or even expected, it follows that any greater nicety of construction here is unnecessary.

The above plan may, however, be objected to, on account of its still leaving something to depend on the elastic force of the air which, should any one consider desirable to be removed, can be so done by adapting metallic valves I I' with projecting points p' p', to strike against the bottom of the barrels, having the spaces O T', O I, filled with oil, to exclude the external air

during their shutting; but even this small addition I consider wholly unnecessary.^^^

Fig. 4. is a perspective view of the one I made for Mr Lees, which is the best method of fitting them up; but the principle is alike applicable to table air-pumps.

Remarks upon the Wasting Effects of the Sea on the shore of Cheshire, between the rivers Mersey and Dee*. By ROBERT STEVENSON, Esq. Civil-Engineer, F. R. S. E., M. W. S., Communicated by the Author.

ON a former occasion, I had the honour to make a few observations, which appeared in the 2d volume of the Society's Me moirs, regarding the encroachment of the sea upon the land ge nerally. The present notice refers only to that portion of the coast which lies between the rivers Mersey and Dee, extending to about seven miles. To this quarter my attention, with that of Mr Nimmo, Civil Engineer, had been professionally directed in the course of last month. In our perambulatory survey we were accompanied by Sir John Tobin, and William Laird, Esq. of Liverpool, along the Cheshire shore, and its connecting sand banks, between Wallasea Pool, in the Mersey, and Dalpool, in the river Dee. Within these estuaries, the shores may be described as abrupt, consisting of red clay and marl, containing many land or boulder stones, of the cubic contents of several tons, and very many of much smaller size, diminishing to coarse gravel. But the foreland, or northern shore, between these rivers, which I am now to notice, is chiefly low grounds and, to a great extent, is under the level of the highest tides. The beach, or ebb, extends from 300 to 400 yards seaward, and, toward low-water-mark, exposes a section of red clay; but, toward high water, it consists of bluish coloured marl, with peat or moss overlaid by sand. This beach, at about half-tide level, presents a curious and highly interesting spec tacle of the remains of a submarine forest. The numerous roots of trees, which have not been washed away by the sea, or carried off by the neighbouring inhabitants for firewood, are in a very decayed state. The trees seem to have been cut off "Read before the Wernerian Society, 8th March 1828.

about two feet from the ground after the usual practice in felling timber, and the roots are seen ramifying from their respective stumps, in all directions, and dipping towards the clay subsoil, They seem to have varied in size from 18 inches to per haps 30 inches in diameter, and, when cut with a knife, appear to be oak. Several of the boles or trunks have also been left upon the ground, and being partly immersed in the sand and clay, are now in such a decomposed state, that, when dug into with a common spade, great numbers of the shell-fish called Pholas candida, measuring about three-fourths of an inch in length, and two inches in breadth, were found apparently in a healthy state. These proofs of the former state of this ebb or shore, now upwards of 20 feet under full tide, having been once dry land to a considerable extent beyond the region of these large forest trees, were rendered still more evident by the occurrence of large masses of greenstone, which, at a former period, had been imbedded in the firm ground here, and especially on the shore within the river Dee. It may farther deserve notice, that the inhabitants of this district have a traditional rhyme, expressive of the former wooded state of this coast, where not a tree is now to be seen, viz. "From Halbre Isle to Birkenhead. a squirrel may hop from tree to tree;" that is from the Dee to the Mersey, now presenting a submarine forest.

As these evidences of great changes upon the state and former appearances of the land were highly interesting to the party, and intimately connected with the professional inquiries of myself and colleague, it seemed desirable to get them, if possible, corroborated by oral testimony. Sir John Tobin accordingly, very obligingly, took measures for examining the oldest people in the neighbourhood, as to their recollection of the former state of these shores. In particular, Thomas Barclay, aged 93" all but two months," by profession a mason and measurer of country work; Henry Youd, labourer, aged 86; and John Crooksan, labourer, aged 80, were examined. Barclay stated, that he had been employed at the erection of the Leasowe landward Lighthouse in the year 1764; that there were then two lighthouses near the shore, for a leading direction to shipping through the proper channel to Liverpool; and that the Seaward Light became uninhabitable, from its being surrounded by the sea. A new light was then built upon Bidstone Hill; and the