This is also a double diagram from the paddle engines of the Great Eastern, taken June 26th, 1860, when she had been nine days out on her first voyage to America.

The pressure on the boiler was 26 lbs. per square inch above the atmosphere, and 21 lbs. were brought upon the piston at the bottom of the cylinder, cut off a little before half stroke, and expanding down the remainder to 4 lbs. below the atmosphere at the termination. On the top of the cylinder the steam exerts a pressure of 30 lbs. at the commencement, and cuts off a little past half stroke. The average pressure was 23 lbs. up to a little past half stroke, and expanding to 2 lbs. above the atmosphere, making only a 9 lbs. vacuum. The exhaust began to close at one end of the cylinder when the piston had to travel 2 feet of the stroke, and at the other end the exhaust began to close when the piston had to travel one foot. It will be perceived that, as the valves are now set, they are exactly the reverse to the position Mr. Scort RUSSEL had them (see Fig. 60, page 354), when on the trial trip in the Channel, so far as regards the admission of steam and the cut off. There are objections to the setting of these valves in both instances; but on the whole, Mr. SCOTT RUSSEL's diagrams are superior. The early closing of the exhaust, with engines travelling only 11 revolutions per minute, is a defect calculated to diminish the power considerably, from the engines having to compress such a great quantity of steam as was left in the cylinder with a 9 lbs. vacuum, acting as a retarding force against the course of the piston. The steam thus left in the cylinder had to be compressed to a greater pressure than that which was in the boiler. The diagram shows that there was a compressed force of 30 lbs., which had to be overcome by the force of the other engine when at its full power assisted by the motion of the ship. If these engines were separated, and not attached to a heavy fly-wheel or some other body in motion, the piston would stop before it arrived at the end of the cylinder, because the compressed or cushioned steam would become of much greater power or pressure than the steam on the propelling side of the piston, or that in the boiler, because the steam left in the cylinder could not escape on account of the exhaust valve closing too soon. This must be a serious drawback to the working of the engines, involving a loss of fuel, besides exposing the engines to the danger of breaking down, by having such a force as 30 lbs. of steam above the atmosphere, upon the crank and piston at plumb centre-almost as bad “as priming" in the cylinder. There need be no wonder at the comparatively slow speed those engines attained. Without taking into consideration various other defects in the arrangement of the valves, there is an almost unpardonable defect which we must notice-the amount of vacuum obtained. On the one side of the piston this amounts to 9 lbs., and on the other 10 lbs., which contrasted with the diagram taken on the trial trip shows a deficiency averaging 2 lbs. per square inch throughout the

stroke. On the one side it is 3 lbs., which proves there must be something requiring alteration. That 4 lbs. of deficiency in the vacuum proves the simple fact that an extra pressure of 4 lbs. of steam is required to give out the power that a proper condensation would have given; in other words, there is a loss of 20 per cent. in fuel, or 20 per cent. in power. The loss of vacuum, and the loss of power at each end of the stroke, accounts, to a great extent, for the slow speed of the engines and the great quantity of fuel consumed. Ex. 4. Work out an indicator card, and show the point where the steam is cut off; whether the slide-valve is correct or requires alteration, and if the latter, what?

Scale of roths to an inch.

From the engines of 8.8. Sea King, or Shannadeah, Confederate Cruiser, Oct. 8th, 1863. Clock & Combrae. Fig. 62.

Top of cylinder

Steam side.

up run

Area of cylinder 1662; I.H.P. 1814.

Steam24 lbs., vacuum 25 in., revolutions 65 per minute.

Top 19.7 lbs. Bottom 24'9 lbs.

Mean Pressure 22.3 lbs.

Ex. 5. Work out the following indicator card, and show the point where the steam is

cut off; whether the slide-valve is correct or requires alteration, and if the latter, what?

The foregoing double diagram is taken from the screw engines of the Great Eastern on January 26th, 1860, during her first trial trip to New York.

The

The pressure of steam in the boiler was 20 lbs., the speed of the piston 285 feet. pressure of steam brought upon the piston at the bottom end was 16 lbs., and at the top

end 18 lbs., with a partial cut off, and the ordinary closing of the valve leaving steam to condense at the end of the stroke, above the atmosphere, and making a vacuum of 10 to 11 lbs. per square inch. The steam being exhausted when the piston has to travel one-seventh the length of the cylinder, that portion, or nearly one-seventh of the power, is lost, the steam being condensed before it has given out its full power. The steam ought to be better used up. There are other minor points in connection with these diagrams that might be remarked upon, such as the projection on the bottom diagram; but as the reader will, by this time, have acquired a tolerably accurate knowledge of the subject, and be able of himself to point out or comprehend such defects, we do not deem it necessary to dilate upon them.

Ex. 6. Work out an indicator card, and show the point where the steam is cut off; whether the slide-valve is correct or requires alteration, and if the latter, what?

Fig. 64 represents two diagrams from the engines of the ironclad S.S. Research. This engine is fitted with surface condensers. The serrated deviation at a is caused by the momentum of the piston of the indicator.

303. On High Pressure Diagrams.-In the case of a diagram taken from a high pressure engine, the whole card would appear above the atmospheric line, as the steam exhausts into the open air, and the mean pressure is obtained from the measure of the depth of the diagram above the atmospheric line. The perpendicular distance between the lower line of the diagram and the atmospheric line indicates the resistance to the motion of the piston, caused by vapour in the cylinder, obstacles to a free exhaust, and the like. The back pressure is this pressure, plus the pressure of the atmosphere into which the steam exhausts.

Explanation of figure.-In the following diagram the piston moves in the forward stroke in the direction shown by the arrow, and backward on the return stroke in the direction shown by the arrow. The steam being supposed let in upon the piston of the engine, presses the piston of the indicator

up to the point shown at the "admission corner," and as the piston moves forward the steam continues to press upon it with undiminished pressure, until close to the end of the stroke, at the "eduction corner," the eduction passage is opened, and as the steam consequently escapes into the atmosphere, there is no longer the same pressure on the spring of the indicator as before, and its piston consequently descends. As, however, the steam cannot instantaneously make its escape, the pressure does not descend quite so low

as the atmospheric line. The eduction passage begins to be opened, as will be seen from the diagram, when about nine-tenths of the return stroke has been completed; and it also begins to close when about nine-tenths of the return stroke has been completed, as appears by a reference to the "compression corner," which shows that the back pressure begins to rise before the termination of the stroke. The area comprehended between the atmospheric line and the bottom of the diagram shows the amount of back pressure resisting the piston, which, in this diagram, is of the average amount of 51 lbs.; and this increased back pressure at the compression corner is produced by the compression of the steam shut within the cylinder, which is accomplished by the piston as it approaches the end of the stroke.

304. On Determining the Mean Pressure from High Pressure Diagrams. -To ascertain the mean pressure we add together the pressure at each division or ordinate, and divide by the number of ordinates. The total amount of the pressures in Fig. 65, taken at ten places or ordinates is 1175 lbs., and the tenth of this, or 1175 lbs. per square inch, is the mean pressure on the piston throughout the stroke.

The following, Fig. 66, is a diagram from the top side of a piston of a high pressure or non-condensing engine, the action of the steam being the same as in the condensing engine, except that the exhaust line is above the

d

Fig. 66.

-39

-38

-22

atmospheric line.

k

The line ab is the atmospheric line; d to the steam line; e to f the line of expansion; at f the eduction valve opens, and allows the steam to escape into the atmosphere. From g to c is the line of exhaust, and the distance from it to the atmospheric line represents the back pressure on the top side of the piston during its upward stroke.

On diagrams from non-condensing engines the line of boiler pressure should be drawn at the top, and it is well to draw the line of perfect vacuum also, that the engineer may be able to see at a glance the quantity of steam consumed, and to compare with it the amount of work done. It is not possible that the back pressure resisting the motion of the piston shall be less than the pressure of the atmosphere, but it may be a great deal more, and very commonly in non-condensing engines the line of resistance is as much as 2 or 3 lbs. above the atmospheric line, though it is quite possible to avoid this excess altogether.

COMPOUND MARINE STEAM ENGINE.

A Compound Engine, called also a High and Low Pressure Engine, is an engine with two cylinders, the one frequently double the diameter of the other. The use of two cylinders, as a means of uniting the full economical advantages due to complete expansion, with the practical requirements of equable motion, is attracting great attention amongst the engineers of the day; and, this class of engine having been tried with the use of high pressure steam, the beneficial results obtained have induced owners almost universally to adopt them for vessels going long voyages; indeed, so great