plane surface of a circular disc; by which arrangement the rapidity of the changes of velocity, due to corresponding changes in the position of the integrating wheel, is diminshed in the same proportion in which the sine of one-half the angle of the cone is less than unity; and the force, necessary to drive the integrating wheel, being diminished in the same proportion, the chance of an error, arising from the slipping of the edge, of the integrating wheel, on the surface from which it receives the impulse, is lessened in proportion.

Thirdly. In the separation of the registering apparatus from the integrating wheel; by which separation, whilst the springs are relieved from the effect of the momentum and the friction due to the weight of the registering apparatus, the latter being in a state of quiescence, the registration is legible whilst the Indicator is in action.

Fourthly. In the variable position of the links connecting the springs together, by which variation the same series of deflexions may be obtained under different ranges of pressure.

In fact, that the Indicator has nothing in common with the "compteur" of M. Morin, except the principle of M. Poncelet, and the springs under a modified form.

The amount of the friction of the pistons was then examined, and the peculiar construction of their metallic packing explained. It was shown also, that instead of great difficulties arising from the friction of the integrating wheel upon the cone, or its slipping upon the surface, a very slight pressure of the spring produced sufficient adhesion to drive the registering apparatus. The Professor then explained the advantages resulting from a registration of the duty of steam-engines generally, not during the time of a few isolated experiments, as with the common Indicator, but extended over any given period, and through every stroke of the engine displaying all the changes which had occurred during that time;-with this view it had been decided, that the instrument should be attached to the engines of the Great Western steam-vessel, on her next voyage to America.

He then expressed his obligation to Mr. Wicksteed for the

facilities afforded him for the experiments at Old Ford, and paid a well-merited compliment to Mr. Holtzapffel for the excellent construction of the Indicator.

In reply to a question from Mr. Vignoles, he stated, that the instrument was not, under its present form, adapted to locomotive engines, but that a grant of £100. had been made by the British Association for the construction of such an instrument.

Mr. Cowper, in compliance with the request of Professor Moseley, illustrated his description by setting the instrument in motion, showing that the registration depended upon the revolutions of the integrating wheel: he demonstrated the cases of motion without pressure, and pressure without motion; in the former case, the integrating wheel being stationary at the apex of the cone, while revolving, does not receive any impulse from the contact with it, and therefore does not register; in the latter case, the surface of the cone, upon which the integrating wheel traverses, being at rest, does not communicate any rotative motion to it, and, consequently, no registration can take place; but when motion and pressure are combined, the cone revolving and the integrating wheel travelling from the apex some distance towards its base, the exact product of the motion of the cone, and the steam's pressure upon the piston, would be registered by the amount of the revolution of the integrating wheel.

Mr. Wicksteed observed, that every facility had been afforded to Professor Moseley for applying his new Indicator, for the purpose of ascertaining the duty performed by the Cornish engine, at Old Ford, but that he had not at all interfered with the experiments, being desirous of ascertaining whether the results would correspond with his trials. That after the work of the engine had been registered, while it was making about 179,000 strokes, the mean result, as stated by Professor Moseley, was so nearly that arrived at by Mr. Wicksteed, that he had no doubt of the accuracy of the machine, as a good indicator of the real duty performed by the engine, the difference, in the result of the mean pressure of the steam, deducting the vacuum, or 0.73 lb., was 0.12 lb., namely, according to Mr. Wicksteed's experiments

12·940'73 12.21 lbs., and according to Professor Moseley 12.09 lbs.; this difference might arise from a variation in the mean length of stroke during the two sets of experiments-from a slight variation in the point at which the steam had been cut off-from a variation in the level of the water in the pump well, or other practical causes, the difference, however, was so insignificant, that he would rely on the accuracy of Professer Moseley's Indicator, and allow the possibility of a slight error in his own experiments.

Mr. Farey observed, that Professor Moseley's instrument must be influenced by variations in the length of stroke, for, whenever the piston makes a long stroke, the cone and the train of registering wheels, must be turned farther round, and would register a higher number than they would do in case of a shorter stroke, supposing the impelling force, exerted by the steam, to be always the same. If the instrument could be really made to give its results, according to the actual length of all the varying strokes made during the time of observation, by truly aggregating these varying lengths into one sum, the results would be free from the usual uncertainty respecting an average length of stroke.

In the monthly reports of engines in Cornwall, the performance is reckoned according to some reputed length of stroke, which had been fixed upon for each engine, when it was first reported; and it is afterwards assumed, that no departure from that reputed length has taken place, when, in fact, such departure does often

occur.

It would be very desirable to have a moving card applied to the new instrument, in order to indicate the impelling force of the steam in the cylinder, by tracing curves on paper, like those by the ordinary indicators. This, it appeared, might be done, with the advantage of causing the paper, on which the curve is drawn, to travel onwards, and bring fresh paper into its place, so as to obtain a series of distinct curves for as many succeeding strokes.

The form of the springs of Professor Moseley's instrument would be a decided improvement, if substituted for the spiral

spring of ordinary Indicators.

Mr. Farey had applied, to an

ordinary Indicator, a mode of exhibiting, at a glance, whether the engine was exerting more or less force than its ordinary appointed task; the plan answered that purpose; but as it required the Indicator to be always in action, the spring of the Indicator broke, after working more than two days,—he therefore abandoned it. The springs in the new instrument were proved, by the trial at Old Ford, to be capable of enduring continual exertion without breaking.

The Professor had stated, that the scale of flexure of the new springs was found to be exactly, according to theory, equal divisions with equal forces; this might be expected, because the flexure of the springs was small, and the bending force acted in a direction nearly at right angles to the length of the springs. In ordinary Indicators, the scale should not always be equal divisions, because the wire of the spring, being wound spirally into a screw of small diameter, the spiral obliquity of the thread of such screw becomes more oblique to the direction of the bending force, as the spring is stretched, and less oblique as the spring is compressed; and hence the scale of pounds per square inch, by which the curve should be measured, for summing up the results, ought to be a scale of unequal divisions.

The Indicators, originally used by Boulton and Watt, were of a large size, with a long and powerful spring, curled into a cylindrical form, as large in diameter as could be included in the cylinder of the Indicator, and the motion allowed to the piston by the spring is very short; such Indicators were judiciously proportioned, and they do not show any sensible inequality of divisions in their scale. But recently, Indicators have been frequently made without the knowledge of their true principle, and the rules of proportion are not observed, so that it will sometimes be found, on actual trial of such instruments with weights, that their scale of pounds, per square inch, is not in equal divisions, although it is usual to employ a scale of equal divisions for summing up the curves traced by them.

In Boulton and Watt's Indicators, the scale of pounds, per

square inch, was formed from actual trial with weights, but such trials were made when the Indicator was cold, and dismounted from its place upon the steam-engine.

A much better mode, is to apply the weights, on the upper end of the piston-rod, when the Indicator is placed on the cylinder of the engine, while it is hot, its piston being supplied with the same quantity of oil, and the spring being in the same state as when it is in use. The depression of the piston, by the weights, is recorded by drawing a line, with the pencil of the instrument, on the cord itself, in the same manner as the usual atmospheric line is drawn thereon.

A series of lines, thus drawn with given weights, become so many original stages for subdividing between them, to form a true scale for summing up the curve described, under the same circumstances, and nearly at the same time.

Professor Moseley's instrument had two cylinders and pistons, operating in concert on the same piston-rod, and springs of peculiar construction to indicate the unbalanced pressure exerted by the steam, to impel the piston of the engine. The elastic force, wherewith the steam acts above the piston, (called the positive pressure or plenum,) is shown by a common Indicator, but the elastic force, wherewith the uncondensed steam is at the same time re-acting beneath the piston, (called the negative pressure or imperfect exhaustion or vacuum) is not shewn; hence the observations are limited to two odd halves of the stroke made by the piston; those halves being commonly the plenum during the descent, and the exhaustion during the ascent of the piston; it is taken for granted that the other two odd halves are the same as those which are observed, although such assumed parity is not always the true state of the case.

In the new instrument, the indication that it would make by drawing on a card, would be that of the difference subsisting between the plenum above, and the exhaustion beneath the piston of the engine, during its descent and ascent; wherefore it would indicate on one card as much as two ordinary indicators can do on two cards, if they are applied one to the top and the other to