glass No. 1, and as the details are the same with respect to all the others, I shall not repeat them. Provide a hydrometer, containing the six numbers, whose accuracy has been proved, that it may be relied on as a standard. With this prepare, before-hand, the solutions of the specific gravity mentioned below. Provide also, four glass jars, ten inches in depth, and two and a half in diameter; a piece of stout wire, about a foot in length; some sealing wax, broken in pieces about the size of small shot; slips of writing paper, half an inch in breadth, and longer than the stem of the glass. Draw a fine hair stroke at one end of each paper at right angles to its length. Fold the papers round the iron wire, to give them a tubular form and facilitate their passage within the glass stem. Make a mark with a fine file at the lower end of each stem to be graduated, about half an inch above the bulb. Fill nearly to the top your four jars, three with solutions of common salt and water, at the specific weights of 24, 16, and 8; and the fourth with pure water, indicating 0, all at the exact temperature of 62. Introduce mercury by a dropping glass into the lower bulb of your tube, till it sinks in the liquid, No. 24, nearly to the mark on the stem; introduce one of the cylindrical papers till the mark on it exactly corresponds with that on the stem. Now cautiously add small pieces of the broken sealing wax, till the paper mark exactly corresponds with the surface of the liquid. With a fine pen make a mark on the paper, on a level with the top of the stem. Withdraw the instrument from No. 24, wash and dry it, and immerse it in No. 16. Draw up the paper till the mark is on a level with the surface of the liquid as before; make another mark on the paper under the former one, and at the same part of the glass. Proceed in this way till you make the mark in the jar containing water, when this part of the graduation of No. 1 is finished. Withdraw the paper thus marked, and transfer the divisions to another piece of paper in a straight line, and have each division subdivided into eight equal parts. Let

the permanent scale be now got ready; it may be about three-fourths of an inch in breadth, and a little longer than the divisions to be marked on it; and let the 24 divisions be laid down upon it, subdivided by dots, and numbered, as shown in fig. 1. Give it a tubular shape, as before directed; introduce it within the stem till No. 24 corresponds with the mark on the glass; secure it in its place by a small piece of sealing wax; replace the glass in the liquid, No. 24, and continue to add small pieces of sealing wax till 24 on the scale corresponds with the surface of the liquid. Close the top of the glass with the blow-pipe, and the instrument is finished. The remaining five glasses are completed in the same manner, only changing the specific gravity of the liquids.

I omitted mentioning in its proper place, that in graduating the permanent scale, the divisions must be inverted; that is, the first divisions marked off at the top of the temporary scale must be made the lowest on the permanent one. The reason is sufficiently obvious.

Solutions of common salt answer well for graduating Nos. 1 and 2, viz. to No. 48 on the scale; but for the other four it will be necessary to have recourse to sulphuric acid.

The most scrupulous accuracy and circumspection are requisite on the part of the artist in preparing his solu tions. It is the most troublesome part of the business, and on its accurate performance his success will ultimately depend. No less than twenty-two graduated solutions are requisite for completing the instrument, one for every eight degrees.

. It is on this account that a correct set of the instruments are recommended as standards, to prevent the vexatious delays attendant on a weighing bottle. The acid solutions should be prepared before-hand, as heat always results from the mixture of water with sulphuric acid d; and when a solution in actual use requires correction, it should be done with acid near its own strength. While

the work is going forward, the temperature of the apart ment and of the solutions should be preserved steadily at 62. Much deviation to either side will produce a corresponding error on the scale. Indeed, while the process of graduation is on hand, the thermometer and hydrometer should be objects of perpetual reference. The operator must be careful to wet the stem of the instrument a little above where it naturally floats, else it will be buoyed up, and the liquid appear heavier than truth; on the other. hand, the stem must not be too much wetted, or the liquid will appear lighter than it ought.

The practised artist will no doubt smile at the long spun detail of trifling minutiæ we have here submitted to. But the recollection of the many mishaps and disappointments that attended my own early essays, induces me to think that small particulars are useful tools in the hands of the young experimentalist.

As much of the time attending the construction of this instrument is expended in graduating the liquids, there will be obviously much economy in preparing a large number at once. It may be well to mention that Ranketli and Agnew, of Manchester, construct these areometers with great neatness and accuracy.

I have never heard that an instrument on this principle has been constructed for fluids, lighter than water; nor do I know any reason why it has not. Besides being much more simple, it would possess other advantages over those in common use. Two glasses, without weights, would comprehend the entire range, from water to the strongest alcohol.

As soon as I can inform myself on this latter branch of the subject, I may take occasion to revert to it.

Reference to the Engraving.

Fig. 7 (Pl. VIII.) represents the scale, before put into:

the stem.

Fig. 8, the entire glass, when finished.

Plan for making a Cut across the river Thames, at the Horse Ferry, Rotherhithe, and for putting down into it a Wooden Tunnel, of 16 feet diameter from cut to cut, and 14 feet diameter within.

BY MR. R. TREVITHICK.

Abstracted from the Papers and Documents of the "THAMES ARCHWAY COMPANY." :

THE cut across the Thames is to be made beneath the water by a steam ballast-raising engine, 24 feet deep below the bottom of the river, and wide enough to receive the wooden tunnel, and with its sides sloped in an angle of about 45 degrees. This cut is to be nearly horizontal at the middle of the river, but declining about 6 inches towards the South, for delivering the water from the road down into the drift; the remaining parts at each side are to be inclined one foot in fourteen, which is about the degree of inclination of the bottom of Holborn-hill.

This slope will ascend to the surface at the South side, about 100 feet South of the shaft, and at the North side about 150 feet North of Queen-street, in the field adjoining to the Commercial Road; making the total length of the tunnel about 2010 feet.

All the earth that is above low water mark may be removed with spades.

The wooden tunnel, for which this cut is to be prepared, is to be made of elm, in lengths of from 180 to 200 feet of six inch plank, placed two in thickness, or in two layers, laid so that the joints shall be covered by the planks in the other layer, fastened together with trennels, hooped outside with iron, caulked, pitched, and made water-tight like a ship. The hooping to be put on in a spiral form, with the spirals two feet asunder.

The ends of each length of the tunnel are to be made to fit into each other, or to be put together with cast-iron ferrules, of 6 feet long, similar to the joints of a flute.

Each of these wooden cylinders will weigh about 200 tons, and may be moved in water nearly as easily as a loaded barge. As many of these cylinders are to be pre

pared as will extend from side to side of the river above low water mark, when joined end to end, which will be about 1340 feet. From each end-of the wooden tunnel to the entrances, the passage is to be left at intervals open to the surface, to admit light, and is to have both its sides and bottom constructed of brick-work, 18 inches thick. This part will extend about 670 feet (at each side), and will complete the tunnel from the surface at one side of the river to that at the other. Staircases for descending into the tunnel are to be formed at each side; the interval of the tunnel between these, which will be about 876 feet, must be lighted by lamps always; the remaining 464 feet (at each side) will receive day light through apertures made like wells from the surface, at intervals of about 30 feet from each other.

After the cut is excavated, piles are to be driven at its eastern side, about 60 or 70 feet asunder, to guide the wooden tunnel into its place. Then the wooden cylinders (which are intended to be made near the Surrey Docks) being ready, are to be rolled into the docks from the banks, and to be towed to the cut, a little before low water, when there is little or no tide, being previously loaded with rubbish sufficient to sink them, but kept buoyant by empty casks attached to them. Here they are to be placed across the river, resting against the piles above-mentioned, their ends to be joined into each other, and to be drawn tight together by a rope or chain, put through them from end to end.

At extreme low water the lashings or cords are to be slipped from the casks, and the cylinders are to be let to sink altogether to the bottom of the cut, which is to be then filled up with strong clay, well rammed down, even with the bottom of the river. A hole is then to be bored into the bottom of the tunnel from the roof of the drift (which is to be previously dug beneath the cut), to let the water down from the tunnel to the well of the steamengines.