such a retardation in its motion as would for ever prevent its again passing beyond the sun's influence. The attraction of a fixed star would accelerate the motion of a comet in receding from the sun towards it, until it arrived at the neutral point, or point of equal attraction of the sun and fixed star; but on the other hand, it would retard its motion on its return towards the sun. Consequently, the time employed in passing from its perihelion to its aphelion, would be less than the time occupied in passing from its aphelion to its perihelion points. Its absolute periodic time would, however, be the same as if it experienced no perturbation from the attraction of a fixed star. Hence, it appears that all the comets which have hitherto appeared to us, belong exclusively to our system.

A body, to perform a circular orbit round the sun, at his surface, must move with a velocity of 987 thousand miles per hour. If its velocity exceeds this, its orbit will be an ellipsis; and if its horary motion amounts to 1,395,856 miles, its orbit will be a parabola; but if greater than this, an hyperbola; in which case, the comet would pass beyond the limits of the sun's attraction, and would not return to our system. Mr. Cole, in his theory of comets, supposes them to pass from one system to another, through the boundless regions of space; but from what has already been said on the subject, as above, this theory appears extremely improbable; for, admitting a comet to have passed its perihelion at the time of the creation of the world, it would still occupy upwards of 3300 times the period elapsed, from thence to the present time, before it would pass the confines of the sun's attractive power. This system appears, moreover, to be altogether inconsistent with the doctrine of universal gravitation. For how, I would ask, is a comet to obtain a greater velocity than that acquired in falling to the sun from an infinite height? "If it passed the neutral point with an excess of velocity, that excess must

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have been acquired in falling towards some other central body of greater attractive power than the sun. But the velocity of a comet in receding from it, cannot be greater than that acquired in falling towards it. How, then, can a comet pass beyond the confines of the system in which it is situated? Its greatest velocity on either side of the neutral point would amount to ouly 470 inches in the first year of its falling from the point of equal attraction towards either the sun or fixed star, independent of the retardation it would experience from the attraction of the sun or star from which it was receding. In fact, its motion in the first year would not amount to a single inch, on account of the opposite attractions of the two bodies. The least possible retardation it might, therefore, experience in its orbicular motion, would prevent its again repassing the same point. It would, therefore, permanently belong to the system in which it was situated."

(To be concluded in our next.)

THE PARALLEL PADDLE-WHEEL DESCRIBED BY CHELMERIENSIS.

Sir-I send you herewith a model of the parallel paddle-wheel, which I have before described to your readers (see Mechanics' Magazine, No. 275, p. 249); it is made by a common tinman, but I hope will be found sufficient to exemplify the principle. The name of the inventor is Mr. Joel Lean, of Fishponds, near Bristol, who is also engaged in the construction of rotary steam engines, on a principle altogether new, and which engineers of the first character have declared to be far superior to any thing of the kind hitherto invented. At some future period I hope to describe this engine to your read

printer's, for the inspection of any of our readers who may be desirous of examining it.-EDIT.]

TEPLY TO LIEUTENANT DODD ON

PADDLE-WHEELS.

Sir-In your 292d Number, I observe that Lieutenant Dodd has compared the passage of a steam vessel to a man walking the streets in London when they are in a greasy state. Now, I confess myself quite at a loss to discover the least similarity, unless, indeed, the Lieutenant wishes us to understand the mud to reach about up to our waistcoats (a consummation which we laudlubbers most devoutly hope may never happen), in which case the resemblance might be somewhat more complete; yet then, our feet would be queer paddle-wheels.

Lieutenant Dodd's communication did not state whether the enlarged paddles were placed upon the same axis as the old ones, or raised higher above the water, which I think by far the more probable, as their curvilinear impulse would thus be lessened, whilst, in the first case, it would be increased, I shall, there fore, assume it to be so, until informed of the contrary.

The reason why the first Scotch steam-vessels worked better with large paddle-wheels than small ones, was not that the paddle-boards travelled through the water more slowly than before; for if Lieutenant Dodd will recollect himself, I believe he will find that their speed was the same; but a less proportion of their diameters being immersed, their impulse was more horizontal, as they did not tend so much to depress the water on entering, nor to lift it when emerging. Another reason is this-when the paddle-wheels were small, in order to make their peripheries move with the requisite velocity, the engines were obliged to work at a rate quicker than was advantageous; it being well known the piston cannot travel with advantage much faster than about 220 feet per minute. Now, suppose the

1.

paddles were enlarged to double their size, it is evident, that to make the paddle-boards travel with the same velocity, the rate of the engines must only be one-half. In proof of this, Lieutenant Dodd well knows that the boat engines of the present day work mostly at their full rate; and, therefore, when the paddlewheels were smaller, the speed of the engines must have been too great to allow them to do their work well.

If Lieutenant Dodd, instead of his muddy comparison of the greasy streets, had compared a steamer to a man swimming, be would have been a little nearer the mark; but as be has not chosen to do this, I will endeavour to do it for him. Suppose an equable force of 20 lbs. will pull or push a man through the water with a certain velocity; now," if he drive the water back with his hands and feet with that force, it is' evident that the same effect is produced as if he were pulled forwards by the 20lbs. But the water yields, and therefore part of the power is lost! The water does yield, truly; but although the swimmer's hands and feet have moved a little through the water, yet be still drives it back with the same force, and consequently advances with the same rapidity, as if he exerted his strength upen a solid medium. And if the power is lost, where, in the name of reason, does it go to? For whatever force is exerted in one direction, must be exerted also in the opposite, let the nature of the resisting medium be what it may, because action and re-action are equal and contrary. With respect to birds, also, their wings strike the air with a force equal to their own weight; and although the medium is far less resisting than water, yet they are supported; and will Lieutenant Dodd say that here is a loss of power? I have frequently rowed a boat in a stream so narrow, that the oars, instead of being in the water, rested on each bank; so that, according to Lieutenant Dodd, the boat, having a solid fulcrum, ought to have advanced with much more rapidity; but this was by no means

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the case, as any person may quickly convince himself by experiment.

A paddle-board containing one square foot of surface, travelling through the water at the rate of 100 feet per minute, would arge a vessel onwards just as much as one containing 100 square feet travelling at the rate of one foot per minute; but the slow motion being the most convenient, it is adviseable to make the paddle boards as large as possible. In short, if a certain force is requisite to drive a steam-vessel at the rate of eight miles per hour, the paddles may be made to exert that force, be they large or small.

I allow that small paddle-wheels are disadvantageous; but I hope I have clearly proved that this is not owing to their driving the water backwards, but to their motion being inconveniently quick, and their impulse not being horizontal. I am, Sir,

Your's, &c.
CHELMERIENSIS.

NEW DYNONOMETER.

A metal cylinder is procured, closed at bottom, and of such length and diameter as the extent of power it is designed to indicate may require. Within this cylinder is a plunger or float of the same length, and less in diameter by about one-twentieth of an inch; this float is of wood coated with wax. At the bottom of the cylinder lies a portion of mercury, about one or two inches in depth. It is evident, that if the float be pressed down, the mercury will rise up in a thin film between it and the cylinder, and that the height to which it rises will give a measure of the force employed in pressing it down. To convert this into a dynonometer, then, we want only a method of exhibiting the rise of the mercury on the outside, and marking the horses' or steam power which operate in pressing down the float. The first object is effected by placing a barometrical glass tube on the outside of the cylinder, communica

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ting by an aperture with the reservoir of mercury in the inside, and in which the mercury will necessarily rise to the same height. A graduated scale is attached to the tube, which expresses the force in pounds, and the divisions are determined by experiment. But the chief merit of this invention lies in a contrivance yet to be described, and without which it would be subject to the same oscillations, and the same uncertainty when applied to ascertain the power of horses, as the spring dynonometer. The glass tube is about one-fourth of an inch wide; but at the lower end, where the mercury passes into it from the cylinder, the aperture is contracted to about 1-30th part of an inch. Hence, the area of the narrow gorge is to that of the tube, as one is to fifty. It will be seen at once, that this contraction of the aperture serves to equalize the effect of the varying pressure of the float. When the horse increases bis pull suddenly, the mercury rises fifty times more slowly than if the tube had not been so contracted; and when the pull is relaxed, the fall is slow in proportion.-Abridged from the "Scotsman," by a Correspondent.

FORCE AND VELOCITY OF WATER.

[Postscript to the Answer by Mr. Baddeley, in our last Number, on the Filling of Syphons.]

The following particulars I have not yet seen explained, in any of the works on bydraulics and hydrostatics which I have met with.

First. The velocity with which water enters a vacuum, and the ratio of the decrease in that velocity, as it approaches that point at which it is in equilibrium with atmospheric pressure.

Second. The force with which a stream or current of water impinges on any given surface immersed in it, at different velocities. And

Third. The question already proposed at page 62, vol. ix.

WM. BADDELEY, Jun.