the upper cross-piece, that unites the piston rod to the vertical bars. The valve rod is to receive its motion from some of the working parts of the engine in the usual manner, and the boiler, and all other parts not particularly described, are also to be constructed as shall seem best to the engine maker. Nothing more being peculiar to this engine, except a pipe for conveying oil above the lower piston, which passes through the case and the lower cylinder near where it joins the upper one, and which is furnished with a cock, and a small upright tunnel at its outer end, to receive the oil.

457. The patentee mentions that the cylinders of this engine and the case may either be all cast together, or in separate pieces, to be afterwards united; and that the partition between the two cylinders, where they meet, may be cast with them, or be made separate and bolted in afterwards.

458. A correspondent in Brande's Journal describes a 'steam engine indicator' which is really a valuable invention. He says 'this instrument which has been long known to engineers, though I believe never publicly described, originated with our illustrious countryman, the late Mr. Watt. That which was employed in our works was made from a description of the instrument given to us by Mr. Field, of London, and was applied with the view of ascertaining in what part the extraordinary friction existed, which had for many months previous prevented the engine from attaining its proper speed.

459. In the first experiments we found, when the whole work was in motion at the usual velocity, that the average pressure upon each square inch of the piston amounted to 11.7 lbs., or about one-third more than it ought to have been; and that at this time the engine, which is of fortyfive horse power, made at Soho, exerted a power, including its own friction, of more than seventy horses. Finding, in repeated trials with detached parts of the work, that the same extraordinary absorption pervaded every department, we became convinced that the effect was produced by some generally existing cause, and naturally directed our attention to the quality of the oil which was in use throughout the work. At one time we were in the habit of using that which is known under the name of neat-foot, but owing to its scarcity, and the extreme cheapness of rape oil, we began, about two years ago, to mix with the former a small portion of the latter. No apparent difference being discovered by the workmen, this proportion was gradually increased, until the stock of neat-foot oil being at last consumed, the rape oil became the sole anti-attrition. And so gradually had the change been effected, that the workmen even to the last denied the existence of any unusual friction, and attributed the want of speed solely to some defect in the engine, which daily exhibited stronger symptoms of being overloaded. At length the use of the rape oil was suspended, and spermaceti substituted; and in twenty-four hours the average pressure was reduced to 9.5 lbs. ; in a week after it bad fallen to 9.1. At this time a mixture of one-third sperm, and two-thirds rape oil was given out to the workmen, and the friction, after

the first day, gradually increased, until at the end of a fortnight the average pressure became 11'1 lbs. A return to the pure sperm oil again reduced the pressure to 9.5. Subsequent observations have given from 8.7 to 9.6, but the inconstant motion of the machinery in a cotton mill must at all times produce such a difference.

460. It has long been a maxim with us, and we believe is pretty generally acted upon by those at the head of manufacturing establishments, to receive and adopt the reports of our workmen upon things of which we think they have, from their different occupations, an opportunity of forming a just opinion: the present instance shows, however, that even in a situation where their daily expenditure of exertion was increased at least fifteen or twenty per cent., they were unconscious of the change, and would only be convinced of its existence by a rapid transition from one extreme to the other. The results therefore of experience, or rather the opinions which men form when, from the nature of the subject, their observations are confined to effects alone, ought to be received with care, and acted upon with caution; and it is only upon the unbiassed results exhibited to our actual observation, through the medium of inanimate matter, acting on known principles, that implicit confidence ought to be placed. The indicator is an instrument of this kind; it exhibits to our view the successive changes of pressure which take place in a steam engine cylinder during each stroke; and, by also marking the duration of each particular pressure, it affords, with an elegant simplicity, a very near and correct approximation to the power exerted. The results which it yields are so tangible, and in many situations so important and instructive to those who have the distribution and application of the power derived from steam engines, that we think it only requires to be more generally known and understood, to be oftener applied.'

461. We may now describe the indicator, plate XI. STEAM ENGINE-A steam engine cylinder cover. B stop-cock, usually made to answer the seat of the grease-cock. C indicator cylinder, about one inch and three-quarters diameter, and eight inches long, open at top, and screwed at bottom upon the stop-cock B.. D a flat pillar, screwed to the side of the cylinder C, and supporting the frame EE. F the piston, fitted so as to work easily up and down, and to be, at the same time, air-tight. EE a frame, twelve inches, by seven inside, the under and upper rail grooved to retain the slidingboard K. G the piston rod, about five-eighths diameter, and sixteen inches long. Ha guide screwed to the pillar D, at six inches above the top of the small cylinder, and through which the piston-rod passes. I a spiral spring attached to the piston at F, and the guide at H. It should be about seven inches long when at rest, and of such a strength as to allow the piston to descend nearly to the oottom of the cylinder, when it is loaded with a weight equal to fourteen pounds upon every square inch of its area. It should also admit of being compressed about one inch and a half. K a small board about seven inches square, sliding in grooves in the upper and un

aer rails of the frame EE. La small brass socket, which may be fixed at any height upon the piston-rod, by the tightening screw M. It carries in the other end a short pencil, with a weak spring to push it forward against the surface of the sliding-board. Na weight attached by a cord to the sliding-board K. O any convenient part of the parallel motion, traversing a space of about four inches and a half during each half stroke of the engine.

462. From this description the principle on which the instrument acts will be evident. By opening the stop-cock B a direct communication is made between the interior of the large and small cylinders, and the density of the steam in the indicator becomes the same as in the steam engine cylinder above the piston. When this density is less than that under the atmospheric pressure, the indicator piston will sink, when it is greater the piston will rise; but the spiral spring, which, if carefully made, stretches through equal distances with equal weights, restrains the motion of the piston; and, by the distance to which it allows it to move from its state of rest, indicates the pressure it is undergoing. During each stroke of the engine, therefore, the indicator piston will rise at the instant the upper steam valve opens, and during the descent of the large piston will maintain a height proportioned to the density of the steam in the cylinder. When the eduction valve opens it will sink, and by the rapidity of its descent, and the distance to which it falls, denote the quality of the vacuum. If, during this perpendicularly alternating motion of the small piston, the sliding-board be made to perform its reciprocating and horizontal course, the pencil, in the socket L, will trace upon the board, or upon a piece of paper applied to its surface, a figure, something like PQRS; of which figure, the part PQ is drawn during the descent of the large piston. At Q, the condensation taking place, the atmospheric pressure acts upon the piston indicator, and makes it descend until the tension of the spring counteracts the force of the pressure. Meanwhile the engine begins to perform the up-stroke, and, as the board traverses, produces the line RS. When the engine piston arrives at the top, the admission of the steam destroys the vacuum that existed below the indicator piston, and allows the spring to raise the latter until the equilibrium is restored. It consequently follows that the distance between the line P Q, and the line R S, will be greater in proportion to the difference between the pressure in the cylinder during the existence of the va

cuum and the pressure of the steam, and the curve QR be more acute in proportion to the rapidity with which the vacuum is formed. If this distance be measured in eight or ten places, and an average taken, then a simple proportion gives the pressure upon each square inch of the piston. Let a area of indicator piston,-b, any weight applied experimentally to that piston,

d, the distance to which it falls with that weight,—and let e be the average distance taken from a diagram, and f the average pressure in pounds upon the steam engine piston during the formation of that diagram. Then, as d to

be

da

b

<- $0 6

is e to f, or =f. And as, for every individual instrument, abd are constant quantities, b then , a constant number by which to da multiply the average distance obtained from a diagram, for the average pressure in pounds upon each square inch of the steam piston.

463. A table of the amount of work really effected by the principal engines in Cornwall in one month may here be furnished. The whole number of engines reported in the month was, fifty-three pumping engines, fourteen whim engines, three stamping engines. Of the pumping engines fifty are single and three double power, and three of the single power engines have combined cylinders; the diameters of the cylinders

as under :

Engines. Inch.

Bushels.

90 of coals consumed in the month 11864

4

41

40

do. do.

36

36 to 34 34 to 30 under 30

do.

do.

do.

do.

duty not reported

do. 90

do.

do.

do.

7

53