that it has increased the power of the engine 11 per cent.

Now we all know, Mr. Editor, that facts are, to use a homely phrase, stubborn things; and to deny the existence of a fact vouched for by parties whose veracity can be trusted, is unreasonable, and can only be the last resort of a worsted opponent's bigotry. But we also know, by every-day experience, that parties are often apt to expect us not only to believe their facts, but also to adopt, as of equal authority, the inferences they deduce from them, which are in a very different category. A dozen people may found as many diverse opinions upon the same data, according to their different modes of reasoning; and indeed it has become almost proverbial, that facts may be made to prove any thing: there scarcely ever was an hypothesis or invention ushered into existence, however absurd or useless, which had not a multitude of facts brought forward to support it; the parties all the while forgetting, that, while the public might admit the fact, they might demur altogether to the inference drawn from it. Whenever, therefore, we find parties ascribing, dogmatically, certain effects to certain causes, we are bound to treat them, not as historians, but as reasoners, and to demand that they show us a legitimate and logical connexion between the inference they deduce and the fact from which they deduce it. Without this, the fact itself, however authentic, proves nothing at all.

So with Mr. Lucy's apparatus; while it would be both unjust and uncivil to accuse our informants of having propagated falsehoods, when they have asserted that Mr. Lucy formerly broke and destroyed his wheels so fast, that, "in 1838, the ruins were strewed about the yard "that formerly he made bad flour, and now he "commands the market"-that formerly he drove nine pairs of stones, and now drives ten-yet we may be allowed to call for something like proof that these effects are legitimately attributable to the substitution of the Isodynameter for the 24-ton fly-wheel, before we are expected to adopt as true such an hypothesis. We will, with all due deference, bring forward some arguments contra.

First, as to the alleged removal of the irregularity of motion. It must be admitted at once, that it is impossible a fly

While the crank is moving from F to B, or from C to E, the velocity will decrease; from B to C, or from E to F, it will increase; at C and F will be its maxima, at B and E its minima. But we are enabled, by a simple application of the laws of dynamics, to ascertain the extent of the irregularity, thus: let x = the space, as a b, moved through by the fly-wheel rim, while the crank is describing any angle, A O B, which make = ☀. If p the uniform effective pressure on the piston, the effect of this to turn the crank is p sin., (disregarding the obliquity of the connecting-rod,) and the Pc sin. effect on the fly-wheel rim

= 19,887

so that the greatest and least velocities during the stroke have a ratio to each other of about 91 to 90; or the greatest difference from the mean is about th of the whole-an amount of irregularity which I think we might safely defy either Mr. Parkes's eyesight, or Mr. Lucy's flour, to discover.

This is the extent of the evil caused by the fly-wheel; let us now inquire how it is removed. Is it by the substitution of an apparatus which, according to the import of its pompous name, "Iso-dynameter," exercises, like water power, a perfectly uniform and equable force on the crank-shaft? Far from it-it does not even profess any thing of the kind. It might easily be shown that the uniformity of force must, in the pneumatic apparatus, be interfered with by many extraneous causes, but this is unnecessary; we will take Mr. Scott Russell's own testimony, as he is very competent authority. His words are, 66 The deviation from the mean pressure is not, therefore, greater than twenty per cent.! and the equalization produced by Mr. Buckle's pneumatic equalizer is as efficient as a pair of engines, and much less complicated and expensive." We need not stop to inquire the effect of this 20 per cent. variation on the velocity, where there is no fly-wheel; but I would ask, is it not ridiculous to expect us to believe that the removal of one regulator, which varies the velocity only per cent., and the substitution of another, which varies the force 20 per cent., should make such marvellous changes for the better? Mr. Parkes is not, unfortunately for him, much in the habit of using mathematical analysis; but it is to be regretted that Mr. Scott Russell, who is one of our most able investigators, did not examine the bearings of the statement he was publishing a little more closely.

The other great advantage stated to

arise from the iso-dynameter is, the gain of 11 per cent. on the power of the engine. It must, however, be noticed, that Mr. Parkes has the glory of this inference from fact all to himself; Mr. Scott Russell evidently gives no sanction to such a notion; he says, "It was found that the change enabled them to give all the grindstones a greater velocity than formerly, so that the quantity ground was greater, in the proportion of 56 to 52." This is a very different thing from Mr. Parkes's assertion; the former merely implies a different and more advantageous mode of applying the power exerted, the latter asserts an absolute gain of power.

Now, if any such gain is caused by the substitution of the pneumatic apparatus for the fly-wheel, it must be either, 1. Because the action of the fly-wheel, per se, causes a loss of power; or, 2. Because the action of the iso-dynameter, per se, causes a gain of power; or, 3. Because the difference between the frictions of the two methods is in favour of the latter to the amount stated.

In these days, when the schoolmaster is abroad, and is teaching the principles of mechanics among other things, we can hardly suppose that Mr. Parkes, although he has some extraordinary mechanical notions (as vide his speech on Professor Moseley's Indicator, passim,) would attempt to persuade us that the gain was referable to either the first or the second of these, for this would be equivalent to contending, that if a hundred apples were sent home from Covent Garden Market, in a peculiarly-shaped basket, they would become one hundred and eleven when they arrived at the end of their journey. The third, then, only remains; and a very little consideration will show that the friction of the equalizing machine is the same in many of its principal items as that of an engine of 20 horses power; and it is for Mr. Parkes to prove his case by showing, if he can, that this, plus the friction of the two spur wheels, is less by 4 or 5 horses power than that which was before caused by the weight of the flywheel.

It frequently happens that the prospects of an invention, which, confined within its proper and natural bounds, might be useful and successful, are seriously injured by the extravagant and unfounded pretensions made for it (frequently to order) by its soi-disant friends.

The present seems a case in point; if theth irregularity caused by the flywheel is a perceptible disadvantage, the use of such a machine as Mr. Lucy's, in conjunction with a fly, as recommended by Mr. Scott Russell, would, probably, not indeed remove it altogether, but considerably lessen it; and if the extra friction were no object, the invention might in such instances stand a chance of being used: but when it pretends to supersede the fly-wheel, to remove entirely the little irregularity it occasions, and to give by some magical process or other an extra 11 per cent. power to the engine, the mechanical public will be very apt to class it with the disc engine, the locomotive momentum, the Cornish percussion, the Boccius gas phenomenon, and other new lights, which Mr. Parkes has of late been so industriously exhibiting to the world.

I am, your obedient servant,
London, October, 1842.

READMAN'S BAROMETER.

π.

Sir,-Reading in your last Monthly Part of the Mechanics' Magazine a description of a barometer lately patented by Mr. Readman, may I beg you to insert the inclosed letter, which was written to the editor of the Meteorological Journal in February, and published in the following Quarterly Journal of April, 1842 ? This document will prove how much of the originality of the invention is due to me, and how much may be the probable value of Mr. Readman's pa

tent.

"To the Editor of the Meteorological Journal.

"Dear Sir,

"I beg to submit the following suggestions on the construction of barometers to the consideration of the readers of your interesting Journal.

"Let the upper four or five inches of the barometer tube be so selected, or otherwise prepared, that each inch of the length shall contain a similar weight of mercury; and let the bore be about half an inch in diameter.

"Let this tube be immovably attached to a wooden frame provided with a scale and vernier, capable of indicating differences amounting to the one-thousandth part of an inch.

"Let the mercury cistern be of iron,

having its interior truly turned or bored of such diameter, that its capacity, with relation to that of the upper portion of the tube, shall be, when taken inch for inch in length, say, as 10 to 1.

Under these conditions it is obvious, that, if the mercury within the tube should fall one inch, that in the cistern will rise onetenth of an inch. But no true indication of the absolute effect dependent upon any atmospheric change can be deduced therefrom without calculation, because the surface of the mercury within the tube, and of that within the cistern, will have approached each other at least the one-tenth of an inch too much; and, moreover, the mercury within the tube will have been prevented from falling so low as it otherwise would have fallen, by reason of the counterbalancing effect due to the improperly raised level of the mercury in the cistern.

"Suppose, however, the cistern to be suspended from the frame of the tube by two spiral springs, (similar to those of Salter's Balances,) so adjusted that the increased weight of the fall of one inch of mercury from within the tube shall depress the cistern, with its contents, just one-tenth of an inch. Then the due compensation will be effected; and this compensation will hold good throughout the entire range of the gra

duated scale.

"As an improvement upon the graduated scale and vernier in common use, I recommend a triangular bar of gun-metal, about six inches long, and graduated into inches and tenths, each tenth of an inch being again subdivided into five parts. Upon this bar, (which may have its three sides each half an inch wide,) a brass vernier, having a "clip" and tangent-screw attached to it, should be made to slide accurately. The vernier should carry a steel index-point; and the divided portion should contain 20 divisions in the space of 19 subdivisions on the bar.

"The bar should be supported in its proper situation by means of two small projecting brass pillars, fixed perpendicularly upon the wooden frame at an appropriate height, and at a distance of about seven inches apart. Each of these pillars should be perforated at right angles to its axis, for the reception of a pointed steel screw with a brass milled head. The precise height of the graduated bar is to be adjusted by means of these screws, whose points should fit into centre punch cavities in the extremities of the bar. By these arrangements, (simple in reality, though tedious to describe,) a very accurate reading may be obtained; and the graduated bar, (which revolves perpendicularly, like a crane, around its centres, when the barometer is held vertically,) together

with its vernier, can be readily removed, (by releasing the screw of the upper pillar,) and applied to any number of barometers similarly constructed, without affecting the original adjustments of any of them, provided the screws in the lower pillars be not disturbed."

"CHARLES THORNTON COATHUPE." "Wraxall, near Bristol, Feb. 18, 1842."

This is the second time that I have freely given to the public inventions that have been subsequently considered worthy of being patented by others. I do not mean to imply that the parties were at all aware of the previous publication of their own ideas, which were, doubtless, as original with them as they were with myself; but I see no harm in claiming one's own, and in presenting what one can afford for the free use of all.

I am yours very truly, CHARLES THORNTON COAThupe. Wraxall, near Bristol, Oct. 13, 1842.

[Mr. Readman's patent was granted in February last; and previous to that date we perused a description of his barometer in manuscript, which differed in no material respect from the specification which he subsequently enrolled. Our esteemed correspondent, Mr. Coathupe, will be quite satisfied, therefore, that Mr. Readman could not have derived his ideas on the subject from the paper by Mr. C. in the Meteorological Journal, which we now republish, since that paper did not make its appearance till Aprilabout two months afterwards. Neither

is the legality of Mr. Readman's patent at all affected by the circumstance of Mr. Coathupe's having published a description of the patented invention prior to the enrolment of Mr. Readman's specification; for, in law, an invention is held to be of the date of the patent, and not of the specification.-Ed. M. M.]

A

W RECISTERED JABY-14842 3

C

B

Sir,-Permit me to call the attention of your readers to the novel and effective arrangement of the coach-wrench, illustrated by the accompanying figure, and recently brought out by Mr. Fenn of Newgate-street. I think most artisans will acknowledge its merits on inspection of the design, but having had it in actual use for some time, I can speak in its favour with the additional confidence gained by experience.

It is, as will be seen by the drawing, an ingenious combination of the lever and wedge, so arranged as to afford the utmost facility of movement, combined with the power of resisting the heaviest strain which the strength of the materials is capable of bearing; these are the essential qualities of a wrench intended to span different sized nuts, bars, or bolts,

and I have no hesitation in saying, that

the tool under consideration possesses

them in a higher degree than any other I have met with of its class.

Description of the Engraving. The cheek A is forged in one piece, with the bar A B, which is cut like a float file on its under side. The travers

ing cheek C, is slotted so as to travel on the square part of the bar, and may be securely fixed to any part of it by pressing the small lever D towards the other portion of the bar, and thereby forcing the wedge W into its seat; this, it should be understood, is done by the act of grasping both parts of the handle in one hand when about to use the tool.

I have no hesitation in asking the favour of a place for this description in your Magazine, because its pages contain ample proof of your readiness to give publicity to useful contrivances.

I remain, Sir, yours very respectfully,
T. B. J.

Pimlico, October 10, 1842.

paper, by which means the light is beautifully radiated and softened to the eyes. If the metallic wick candles are used, there will be no necessity for the removal of the shade from the gutter or rest for snuffing. A slight-tinted paper of a lilac colour has a very beautiful effect; the paper should be previously moistened with a solution of borax, as it will prevent its taking fire.

As this is the only method by which a self-acting shade can be attached to candles used in common candle-sticks, it is likely to be serviceable; or for workshops, I would greatly recommend the use of Mr. Thomas's Candle Guards as saving a deal of otherwise wasted tallow. I remain, yours obediently,

CHARLES CROYDON.

P. S. In the prefixed figure the shade is supposed to be transparent for the sake of better showing the interior. Devonport, October 12, 1842.

Sir, Having derived great benefit from the use of a shade over the common candle, of a new construction, you would be doing a service probably by making it known through the medium of the Mechanics' Magazine to that portion of your readers who may have occasion to read or write much by candle-light. For the method of suspending the shade I am indebted to the invention of the "Candle-Guard" of Mr. David Thomas, 18, Goswell-road, for the prevention of guttering in candles, for which he received the Silver Isis Medal from the Society of Arts.

A is a cone of glass with a hole for the wick to obtrude through, which, by projecting over the top of the candle, forms a cup to retain the melted grease, and prevents it from guttering down the side. This glass cone is kept in place by a ring of glass or metal B B, suspended by three chains or cords E E E, to the cone, which, as the candle burns, causes the cone of glass to descend evenly on it.

Observing the great use of this in currents of air, I made one, but instead of the solid ring of metal, or glass, I had a hollow gutter with the chains attached to the inside edge. I then formed a light wire frame, and covered it with thin

ZANDER'S EXPANSION AND CONDENSING

SYSTEMS.

Sir, I beg to thank you most sincerely for the favourable account of my steam-engine improvements given in the last number of your valuable Journal; and also for the insertion of the principal items in my Comparative Table of the Dimensions and Performances of the Era steamer and her six competitors. But, as I wish the principles on which this Table has been calculated should be well understood, I have to request you will do me the additional favour to insert a few words in explanation of the same.

The elements which we have to take into consideration, when calculating the motion of a vessel through the water,

are:

1. The friction of the exterior surface of the vessel immersed in the water.

2. The resistance of the vertical midship sectional area in the water.

3. The angle of incidence the vessel has at head and stern.

But, as the seven vessels included in the Comparative Table are nearly all alike in form at head and stern, we need only take into account the first and second elements.

1. The way in which the friction is calculated is this :-The resistance caused by the friction which the surfaces of dis[See p. 394.