the slide passes through with a thumb screw I, to fasten the sliding piece, as is more fully shown in fig. 2, where the braces cross or meet each other.

K, fig. 1, represents a brass rod with large milled head, suspended to the front of stage by a bracket and socket, and terminating with an universal joint; to which is attached a pinion-wheel fixed to the underside of the radius arm, in such a manner as to act upon the teeth of a sector firmly screwed to the front of the stand, and shown by LP fig. 4. By turning this rod the pinion acts upon the sector, and carries the instrument with a steady horizontal motion.

ST, two sliding brass rods to steady the tube, and free it from vibration; they are in two lengths, one sliding within the other, and are attached to the high end of the tube, with a double joint, and to the cross part of the radius arm near the feet of the stage also, by joints as more fully shown in Fig. 2, S S.

R, a brass plate, and dove-tailed slide with eye-glasses, and arm carrying the small speculum as shown on a large scale at fig. 5 and 6. The slide is moved by the screw with milled head, to adjust to the focus of the eye-glasses.

W, a small telescope finder fixed to the tube, and adjusted parallel to its axis, to enable the observer to find his object in the field of view of the large telescope, with greater readiness.

From the preceding description it will be observed that this instrument has two vertical motions; viz., that of the slide piece E, used where extensive changes of altitude require a speedy elevation, (which can be held in any position by the thumb-screw I); also that of the screw G for the finer adjustment. A horizontal motion is obtained by turning the brass rod K. In using this instrument it is necessary to place the tube, pointing as near as possible to the object, then by using any or all of the above-described motions; bring the object into the centre of the cross wire contained in the finder or small telescope; and as its axes are adjusted parallel with the large tube, the object is in the field of view there also; and from the complete steady. command over the instrument can be easily retained in that situation.

EXPLOSION OF THE "VICTORIA" BOILERS EXAMINATION OF THE PRINCIPLE OF THEIR CONSTRUCTION.

Sir, I have thought much of the Victoria flues; and it appears to me so extremely probable, that such flues as they are described to be, will collapse sooner or later, even if a full and sufficient quantity of water is kept in the boilers, that I have determined on publishing my opinion, and the reasoning by which I have arrived at that conclusion.

If we were to put an elastic fluid into a flexible cylindrical vessel of equal strength and thickness all over; and then by extraneous force alter the form of the vessel; the pressure of the fluid would tend to restore it to its original shape.

If we subjected such a cylinder to the pressure of an elastic fluid on the outside, it would bear any force less than one sufficient to crush the material of which it was composed. But that is only on the supposition that the form is perfect, and the strength and texture exactly alike in every part. For if its form were altered, although in ever so trifling a degree, by any extraneous force, the pressure of the elastic fluid would flatten it, or crumple it up, or tear it, according to the direction of the extraneous force, and the nature of the material of which the cylinder was made. And the same effects would follow from some parts being weaker than other parts, although no extraneous force were used to change its form.

These are properties which belong to the figure, may be demonstrated, and I suppose will not be disputed. The following observations apply to circular flues having the pressure outside.

I shall state three propositions, the truth of which I think cannot be doubted by those acquainted with the subject.

1st. The flue is not, and cannot be made, of exactly equal strength in all its parts.

2nd. Its form is not that of a perfect cylinder, but a rough approximation to it; it is full of dents and irregularities.

3rd. Any given flue may be placed in such circumstances of heat, and pressure, as will render it flexible.

Now we know that circular flues are made, and are placed in such circum

stances, that although they are not inflexible, strictly speaking, their original form never suffers any permanent alteration. Hence it appears, that the degree of stability of any given circular flue, depends upon the degree of heat and pressure to which it is subjected; upon the extent of the deviation of its figure from that of a perfect cylinder; and upon the difference in the strength of the different parts.

But there is yet another cause of weakness and of change of form-the unequal heating of the flue. The flame and hot vapour will always act more directly, and with more effect upon some parts of the flue, than upon other parts; unless the flue is very small.

steam =

Now let us imagine a flue to be of such strength and form, and in such circumstances, that when it is at the temperature T, and the pressure of the P, it suffers a very small permanent change of form; and that the temperature and pressure at which it is commonly worked, are very nearly and P; and consider the consequences that are likely to ensue.

=

T

By the fortunate combination of a variety of different circumstances, such as the water used leaving little or no deposit the flue never getting hotter in some parts than usual from shortness of water, alteration of furnace, or any other cause the steam never getting to so high a pressure as that we have taken as the limit of the flue's strength, &c., such a flue might last a considerable time; in fact, until it got sufficiently weakened by fair wear and tear, to allow of ordinary circumstances changing its form, when it would collapse, and probably cause loss of life. This seems to have been the case with the flue of the boiler on board the James Gallocher. See the Times of June 30.

A flue of this kind may be placed in such circumstances, that when the steam and temperature get unusually high, or as high as the safety-valve will permit, it suffers a minute permanent change of form. Under these circumstances it will inevitably collapse sooner or later, according to the extent to which its form is altered at each time it is unusually heated, and the frequency of that occurrence, let it be surrounded with as much water as it may and I have not the least doubt, that both the Victoria

flues have collapsed from this cause alone. I state this opinion with all due deference to that of Mr. Seaward; but that gentleman says he could not satisfy himself that the flue had been red hot; and that, although it opened at the bottom, the mischief might have begun at the top. Now it appears to me, that if the top of the flue had been softened by heat, so as to cause the immediate failure of the flue, that it would have been forced inwards, and the flue would have failed in that part: or if not, that it would have exhibited a very sharp bend outwards, and thus have proved its having been overheated in that particular part. All the other circumstances which would occur in the case supposed by Mr. S., seem to me very accurately described.

I will not occupy more of your valuable space in considering what would happen to these flues under the various circumstances in which they might be placed. But I must observe, that if we wanted to know what such flues would bear, the only way we could ascertain it would be by actual experiment. Such experiments have, unfortunately, been tried on a large scale. We know that each of these flues that has failed has exhibited certain peculiarities in the circumstances of its failure; and our theory has pointed out the particular circumstances that would lead to such peculiarities as have been exhibited in each of the failures we have spoken of, and would do so by them all. Have we not therefore a right to conclude, that the circumstances we have supposed, and those which actually occurred, are precisely alike?

I must observe, that it is totally impossible to ascertain the strength of these flues by calculation, and any pretence to do so would be absurd, or something worse; because, to make such a calculation, we must have the exact form, strength, and temperature of every part of the flue when at its highest temperature. And that the flue which would be safe with one furnace, might very likely fail, in a short time, with a different one; although the weight on the safety-valve remained the same.

But is proposed to prove the remaining boilers of the Victoria and thereby establish their character for safety. How would they prove them? Doubtless, by the same means they have proved the whole four-HYDRAULIC PRESSURE.

.

Now really, if ignorance were not the best excuse for this proposal, and therefore, that it might seem uncharitable to believe the parties knew better, it would be out of my power to suppose they did not. According to their own account of the matter, the water in two of these boilers has been allowed to get too low, although they were under the care of most experienced engineers, who well knew the consequences of such inattention,-and the last of whom could not have forgotten the dreadful punishment the first victims suffered for their alleged neglect. It is therefore sufficiently established, that these boilers, from some cause or other, are liable to this accident; and one would think the advocates of these boilers must know, that after the flue has once been overheated from this accident, and its figure altered by this or any other cause, that its strength is lessened; and that under such circumstances it is monstrous, to talk of the hydraulic pressure it has sustained, being any assurance of its present safety.

I have written thus much, because I hear other boilers are making on the same plan, and I feel very desirous of warning all concerned of their danger. But I trust no more lives will be sacrificed to an obstinate perseverance in the use of these most incongruous boilers; which will most certainly be the case if their use is persisted in.

Yours, &c.

C. G. JARVIS, Engineer, &c.

July 14, 1838.

P.S. Fourteen people have been killed by the collapsing of these flues; and yet, a certain person has deemed it wise to declare, they are "the very best boilers that can be made;" and that "the Company are more than ever convinced of their safety," notwithstanding the fact of every engineer, except the unfortunate person who designed them, condemning them as highly dangerous ?

It certainly behoves all persons who have any regard for their lives, to be very cautious how they entrust themselves to the care of the servants of a Company" of persons whose judgments are so singularly and so conspicuously perverted.-July 19.

Your intelligent correspondent "N. S." page 265, says, "the enginemen must keepa bright look out for themselves," &c.

I am decidedly of opinion that all pos sible attention could not have prevented the collapse of the Victoria flues.

High steam, is as safe as low steam, when those who construct and manage the apparatus for its application, know all they ought to know, before they meddle with it. But if any company should choose to employ any one who is not fit for the task, the credit of that company must suffer, and the safety of the public will be endangered. C. G. J.

MR. WALTER HANCOCK'S, AND SIR JAMES ANDERSON'S PATENT BOILERS.

Sir, To the Mechanics' Magazine, more than to any other journal, is the cause of steam-locomotion on common roads indebted, not only for its liberal advocacy, but its impartial insertion of the performances, plans, and pretensions of all those who have, in the course of their career, made out any tolerable title for such a distinction, admitting at the same time, through the medium of its columns, a clear stage and fair play" for such as may have misrepresentations to complain of, or explanatory communications to offer; thus affording to the scientific competitors of the present day an arena more facile and available than has fallen to the lot of their predecessors in any age or country.

Such, Mr. Editor, having been your course hitherto, I trust you will not deny me an opportunity of answering a letter which I have received from one of your readers (besides various verbal interrogations) respecting a boiler, described in your 775th Number, purporting to be the patent of Sir James Anderson, and to form the principal feature in a steamcarriage for common roads, building or built by him for a public company. I have examined the wood-cut and description referred to, and find that the boiler is composed of flat chambers, for which invention I obtained a patent in the year 1827; and, as I have never granted a license to any person whatever to use my said invention for steam-carriages, no person can have a legal right to adopt it.

I should not have taken any notice of this circumstance if I had not been called upon to do so; because, until a boiler