has the line o, which pulls the triggers, on that side. m2 has only one shaft, P, which cocks the locks on the left-hand side. It will be seen that the locks, &c., on this side, are of the same construction as those on the other side. Now, suppose a horseman arrives at the gate; first his horse treads on the plate m2, which by that means cocks the triggers on the lefthand side (which have been left fired by the preceding vehicle); next he puts his money into the funnel c; the rod ƒ sinks, and pulls the line l; the lowermost lock is thus discharged, and the lowermost rod 4, is driven into the partition on the lefthand side, thereby leaving room for the horseman to pass underneath. On the other side of the gate, he treads on the plate 7, and so cocks the lock on the right, which he before had discharged, and fires one on the left, to drive the rod 4 back into its place. As a gig pays double what a horseman does, two of the rods would be thrown across, and so on. it is worked by weight, of course one penny must be put in. Such is the Selfacting Toll Keeper. I hope I have made it sufficiently intelligible.

I am, your constant reader,

Θεωνδωρον.

ATMOSPHERIC PROPULSION.

As

Sir, In the present volume of your Magazine, page 85, you have inserted some "Notes of a Mechanic," the first of which suggests a supposed new mode of atmospheric propulsion. Will you allow me to inform your correspondent, through your useful pages, that a system of atmospheric propulsion, almost identical with that which he proposes, was patented by Mr. Pinkus in, I think, the year 1842. The only difference which I at this moment recollect was, that the piston cylinders, upon being placed by the passing train in communication with the main tube (and therefore in action), communicated their power, not immediately to the train, as your correspondent suggests, but through the medium of wheels, which they caused to revolve, and which then, either by their frictional hold on a bar, or by cogs on a rack attached to the carriages, propelled the train. The last carriage of the train again shut off the communication between the cylinders and the main tube.

I think I see in the tone of your correspondent's remarks enough to assure me that he will rather rejoice than be disappointed in finding that others have thought of the same thing as himself, and that he knows and appreciates the saying which explains

MODE OF CHECKING ATMOSPHERIC RAILWAY ENGINES IN DESCENDING INCLINED PLANES.

Sir, The principal objection to the atmospheric system of railway propulsion urged by your correspondent, Mr. Lipscombe, appears to me easily remediable, viz., that no unobjectionable means have been proposed to avoid the propelling power during the descent of an incline. I apprehend that the following method will obviate the difficulty:

We will take a tube of three miles in length, in which, at some intermediate part, there is an incline, wherein safety compels the cessation of propelling power. The main tube may be continued the whole length, but remain open the distance of the incline. Let a closed tube of equal diameter diverge at the summit, and run parallel with the main tube, and be also in connection at the base of the incline. A revolving double valve-similar to those already used on atmospheric railways-is to be attached at each junction of the double tubing. The piston, on arriving at the upper junction will close the valve of the secondary tube, at the same time opening the partition in the main tube. The train will then descend the incline in connection with the open tube; consequently, no power will be applied. On arriving at the base of the descent, the revolving valve at the lower junction will again be opened by the piston, and the air being admitted into the secondary tube, the valves will resume their original position, the train proceeding as usual. This may be repeated several times between two stationary engines.

I am, Sir, your obedient servant, Cornhill, September 1, 1815.

R. W.

round, it was not possible to weigh the coal consumed.

When the Great Britain was commenced, the city of Bristol had taken up the subject of widening the dock-gates of the port, with other improvements, so warmly, that no doubt was entertained that, before she should be completed, there would be no difficulty in her going out; accordingly she was designed 5 feet 6 inches wider than the existing locks.

Various causes led to the abandonment, for a time, of these improvements, and the ship, when ready for sea, was not only discovered to be a prisoner, but likely to continue so, in consequence of the personal liability which it was assumed the Dock Company might incur if, by permitting any disturbance of their works, not provided for

by Act of Parliament, any injurious consequences should ensue to the port.

This state of affairs lasted for several months, until at length, by an agreement between the two companies, permission was accorded to remove, first so much of the masonry and gates as would allow the ship to pass from the floating harbour into the outer basin, next to restore these, and then to adopt the same course with the gates and one side of the lock communicating with the river Avon.

This was accomplished, and the ship hauled out on the evening of the 11th of December, and at 8 o'clock on the following morning she was towed down the river Avon to Kingroad: the boilers were filled in the progress, the steam was raised, and a trip of a few hours' duration was made, the greatest speed then attained being

Feet.

Speed of the Ship ...... 11 knots

The next trial was on the 8th January, when a numerous party of proprietors, and several engineers and scientific men were on board; but unfortunately the fog was so dense, that after waiting at anchor for several hours, the pilot, apprehensive of losing sight of the land, reluctantly consented to go a short distance, merely to gratify the visitors.

On this occasion the greatest speed of the engines was 184 strokes, the speed of the ship was 11 knots, and the slip was 13 per cent.

On the 20th January a run was taken down the Bristol Channel, nearly to Ilfracombe and back, a distance of 95 knots, without much wind, but in a head swell, and with a balance of about two hours of tide against the ship. This distance was performed in 8 hours and 34 minutes, or at an average rate of upwards of 11 knots.

The greatest rate of engines was 183 strokes per minute, the steam pressure being 2 lbs., and the vacuum 26 inches, and cutting off at 18 inches of the stroke; when the ship's speed was 12 knots, the slip of the screw being 9 per cent.

Finally, the Great Britain quitted the port of Bristol for London on the evening of the 23rd of January.

The masses of cloud which had traversed the sky during the day, and the occasional heavy gusts of wind, indicated the coming of the gale, which was shortly after experienced, as will be observed in the summary of the voyage (for which, see next page.)

{Velocity of Propeller

through the water.

82.10 Slip, or as ⚫93 to 1.

During this voyage the engines made 52,773 strokes, consequently the distance described by the screw was 639 knots, and the actual distance traversed by the ship, as computed by Captain Hosken, was 567 knots. The ratio of the speed of the ship, to that of the screw, during the entire voyage, was as 887 to 1; or in other terms the total slip was 12 per cent. Considering then, that during the first 20 hours there was a strong gale and a head sea, and also, that in the run from the Downs to Blackwall, there was an exceedingly stiff head gale, while in the intermediate part of the voyage the wind was so light as to be of little service, this may be accounted an exceedingly favourable result.

The balance of tides was also considerably adverse.

The time the ship was under weigh was 59 hours, so that the average speed was upwards of 9 knots; and if allowance be made for times when, on account of the bearings becoming warm, the engines went slowly, the average speed may be fairly reckoned at 10 knots per hour.

Owing to the inefficiency of the stokers, the steam was not regularly or well kept up, and the pressure varied from 2 lbs. to 5 lbs., being frequently low. Duffryn coal was used, and all the ashes were burned. The throttle valves were kept more than one-half closed, and the expansion valves cut off the steam at one-sixth of the stroke, so that the economy of the fuel must have been very

tom, the centre of gravity was raised so high that the rolling, which was considerable, but very easy, is not surprising.

With the wind a head, or on either bow, and with a heavy head sea, she steered with the greatest ease and precision, and in the crowded river it was truly surprising how she threaded her way.

When the heavy sea before mentioned struck her, it caused no deviation whatever from the uniform motion of the engines, which went on as steadily as if they had been on land, neither was there the slightest. yielding in the plummer-blocks, the frame, or in any part of the engines, or the engineroom, which is so riveted together, as to form one united frame.

On several occasions the author watched the screw, and he does not think it ever rose one-half of its diameter out of the water; and, standing by the engines during the worst of the gale, he could only observe that there was occasionally a slight acceleration, during perhaps half a revolution, but there never was any check to the uniform rate.

The paper is illustrated by seven drawings and diagrams, Nos. 3778 to 3784, showing longitudinal and transverse sections of the vessel and engines, with an elevation of the screw-propeller and diagrams of its angles of pitch, slip, &c.

applied, was not from a fear of breaking the screw, but because of its small dimensions, it having been made for the 'Archimedes,' whose engines were 80 or 90 h. p., and more particularly on account of the pitch being so small. The result was, that when it was applied on board the 'Napoléon,' the engines (which were 130 h. p.) would have required to have been driven at such an increased number of strokes, that the boilers could not have supplied sufficient steam. Even with the throttle-valve partially closed, great attention was required to keep up a steady speed.

He thought the results obtained, with such a small propeller, quite extraordinary, and such as could not have been anticipated. He had since calculated the results more accurately, and found them as shown in the following table :

Mean 32.8

The speed of the screw of 7 feet 7 inches pitch was 10.7165 knots, which was in the ratio of 0.9047 to 1.

In his original calculations, given to Mr. Guppy, he had erroneously assumed the pitch of the screw to have been 8 feet. 10.15 lbs.

Remains

Mr. T. R. GUPPY stated, in answer to questions from the president and members, that during the whole voyage the throttlevalve was only one-third open, and that the steam was cut off at one-sixth of the stroke. It was not possible to take any accurate account of the coals consumed, but he estimated the consumption at about 40 tons in 24 hours. The screw propeller of the Great Britain was too small, but still the speed obtained even against a heavy head sea, was never below 5 knots per hour.

CAPTAIN HOSKEN said, he considered it necessary to reduce the speed in heavy wea.

1.55

per square inch.

8.60 95.5 h.p. for each engine.

ther; it was in such cases dangerous to ap ply all the power of the engines; there was a danger of the sea making a clear breach over the vessel if she was driven bodily forward, instead of being allowed to rise with the waves. The experience he had acquired in the Great Western had clearly proved the correctness of his views.

His opinion of the advantage of the screw as a mode of propulsion was decided, and he thought that it would, for sea-going vessels, supersede paddle-wheels. During the worst part of the voyage, with the Great Britain, the screw was never more than one-half of