The most obvious plan is an inclined plane; the waggon full of ore descending, to draw up the ascending empty one.

Fir timber is to be obtained at a moderate rate; would not, there' fore, a tram road of balks answer equally well with an iron one?

If so, would a different kind of waggon from that usually employed be necessary? Must the wheels be broader? or would trucks, without wheels, answer better than waggons? Would a half-inch chain let down three tons of ore? What would be the size of a rope to correspond? And how long would each, probably, last, supposing 3000 tons were the quantity to be conveyed annually ?

What kind of machinery would be necessary in order to regulate the velocity of the descending waggon? Would a double chain or rope be necessary?

After the ore is shipped it is to be re-landed, in order to be crushed; the distance from the landing place to the crushing-mill is 50 yards; the angle is 15 deg. It is proposed to employ the water-wheel of the crushing-mill to draw up the waggon full of ore.

What weight would a half-inch chain draw up? And as the friction caused by the loaded waggon ascending would be much greater than in conveying the ore from the mine to the water's edge, where the loaded waggon descends, would it be necessary here to have an iron tram road? or would a wooden one also answer here?

SOCIETY FOR THE DIFFUSION OF

USEFUL KNOWLEDGE.

Sir, I am entirely of your opinion, that the discussion in reference to this new Society has been carried quite far enough to answer all the purposes of fair inquiry. And as I have no doubt that the majority of your readers perceive that "Mentor" leaves the citadel of my main and original statements (untouched, as well as most of my subsequent reasonings, while he diverts himself with some skirmishing at the least

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important of the outworks, I may very quietly leave much that he advances in his last letter without reply. Regarding, then, as I trust I now may, my first position with respect to the Society as established, I shall merely notice a few particulars, in which "Mentor" either calls in question my assertions, or seems to intimate a want of consistency and fairness in the course I have pursued.

The charge of inconsistency, however, as well as that of unfairness, may at once be disposed of; for they hang merely upon this, that I have advanced statements, arguments, and opinions, in my later letters, that I did not specify in my first; as if the essence of discussion consisted in reiterating the same things, and scrupulously avoiding the mention of any fact, or the developement of any sentiment, in a reply, or a series of replies, but what had been adduced in the outset. Very amusing, truly! and very convincing! "You said so"—"but I say so," from the beginning to the end of the chapter.

Thus, I have praised some of the Society's pamphlets, and I have found fault with others. But, since this does not all occur in one letter, "Mentor" very gravely informs us that this "one circumstance struck him as singular." In my latter letters, too, I speak of the influence of the Society, which "Mentor" regards as very unfair; for in my first letter he does not find the word influence. It may be so. But, in my first letter, I said that "the direct and constant tendency of the Society was to check the spirit of enterprise," &c.; and how, when this is denied, or attempted to be denied, I could prove the tendency to produce the injurious result, without showing that its influences inevitably involved it, it is not for me to say. Perhaps, when "Mentor" publishes the Society's Treatise, of LOGIC, upon which, I presume, he is engaged, he may favour us with a few corollaries, in which all this may be made very obvious.

My assertion was, that in two years the Society had delivered to

tical (as A and B, fig. 5), at others horizontal (as at C and D), with every possible variation between them, to an infinite number of radii.

This kind of parallelism will never be of any service in practical scribing, because all stuff, when scribed, can be moved up in one direction only; hence, a concentric circle, thus perfectly parallel, when moved up, instead of fitting, will only touch in one point; and all the horizontal spaces or distances will be different, except their opposites above or below (as in fig. 5, lines 0 y≈).

On the contrary, in practical scribing, when the stuff is cut to the same height, as, for example, a wall, and brought up to touch, then the points of the compasses, instead of moving in the way described in the parallel concentric circle above, must be held with both points constantly parallel to the horizon, and no other; that is, parallel to the direction the stuff is to be moved up to; and if this is done correctly, every horizontal line of distance between them will be of the same perfect length; which I trust will clearly explain and prove my former assertion.

In the following example let C

represent a wall, and D the board to be scribed to fit it. Then, if the points of the compasses begin at the top, one foot at the edge of the wall, and both feet be kept paralle to AB, -then, I say, it is all which is required to scribe perfectly, and, of course, nothing can fit better; but should the points of the compasses once diverge from their horizontal position, the scribing of the board

will not be correct, and consequently will not fit. The common mistake of scribing is, that when one point of the compass comes to a curve, instead of keeping the other point opposite constantly horizontal, the operator supposes the true opposite point is not horizontal, but on the radius of the concentric circle of that curve; which, if done, is erroneous, as ef instead of gf; and

this not fitting, though much approaching, is the reason why it has, by such error, to be scribed a second time. In this way the error diminishes, certainly, every time; but so long as this principle is practised, it is erroneous at last.

I do not say that Mr. Curtis's instrument will not scribe truly; on the contrary, if the two points are steadily fixed, and horizontally kept in their motion downwards, the correctness will be the same; but what I said and meant was, that the common compasses, if held in the same position, would do the same thing. I consider Mr. Curtis's instrument as a compass of a different shape, differing as much as the common beam and gauge; in some instances being kept steadier, in others not; and its opening as a sliding gunter, instead of an axis. Nevertheless, to some persons the one might be more handy than the other; and if so, might be used with advantage accordingly; but I still consider the principle exactly the same.

It must be decided by others whether fig. 4, in p. 200, No. 244, is correctly scribed. I say that it appears not; for the top and bottom of the two substances already touch, and the middle does not; then, consequently, as they are, cannot fit; for when one part touches, it cannot be moved up closer. The figure is"thus :

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irregular curves will, if continued, often intersect each other, I remain, Sir,

Yours, &c.

S. C. OBSERVER.

P. S. In Mr. Curtis's fig. 3, one of the lines should have been dotted, to show the true one; and in fig. 4, letter C is a misprint for G. The instrument would have been much better explained, I think, if he had shown the two scribing points at the extremity of the blade, which do not appear in the figure. Also, a straight edge I should think inconvenient to be placed to scribe against a wall, which is the most common of scribing cases.

THE LARGEST STEAM VESSELS IN ENGLAND.

Sir,-Being only an occasional reader of your Magazine, it has but lately come to my knowledge that you have stated, as a quotation from my Treatise on the SteamEngine, that an engine erected by Mr. Watt, at Hawkesbury Colliery, is the largest in England—(see your 9th vol. p. 336). Also, in your 10th vol. p. 304, the same statement is repeated, as being on my authority; and you have added some particulars from Cornwall to contradict it.

I wish to inform your readers that the statement in my Treatise really is, that, in the year 1778, the engine in question was supposed to be the most powerful in England. At p. 338 of my Treatise, the date 1778 is given in a computation of its performance, and is followed by these words;-"This engine must have been nearly 76 horse-power. It was considered to be the most powerful in England at that time; for it was capable of performing more work than some of the old atmospheric engines, with cylinders of 70 and 72 inches diameter, and that with less than half their consumption of fuel."

As a fact respecting the power of steam-engines existing in the year 1778, I know my statement will