diminished, but this remedy is very objectionable, and should be applied as seldom as possible.

When there is not enough steam, the draught through the fire, in consequence of the low pressure of the steam, and the slow motion of the engine, will necessarily be less energetic than it ought to be, the means of exciting the fire becoming inefficient at the time when its assistance is most wanted. A good engineer will certainly take care that this occurs as seldom as possible, but there are accidental causes over which he has not sufficient control, and on such occasions the power of contracting the orifice of the blast pipe would be very beneficial, by enabling him materially to increase the rapidity with which the fire would be brought up to its proper state.

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A

By good management, the engineer can therefore have full power over the production of steam, so as at all times to have a good supply, and to prevent almost entirely the loss occasioned by its escape from the safety valves while the engine is in motion; and taking into consideration the frequent occasions on which advantage may be derived by varying the orifice of the

blast pipe, it may be inferred that it is as requisite to have full command of this orifice, as it is to be able to determine the position of the regulator. The speed of the engine may, moreover, be occasionally regulated with advantage, by varying the orifice of the blast pipe, without altering the position of the steam regulator. To carry out, in a practical manner, the variable contraction of this orifice, it is requisite-That the apparatus should be easily constructed and applied, and not liable to get out of order; that its action should be simple and effective; that an indicator should show the area of the orifice under which the engine is working.

Having pointed out the general advantages I propose to derive from the application of a variable blast, I will now describe the apparatus that has been employed, which will be clearly understood, with the assistance of the annexed figures.

In the construction of this variable blast, there is one point on which it is proper here to make a remark, which if not attended to, would materially tend to destroy the good effect to be produced.

The annular space between the internal cone and the orifice of the blast pipe, if too much contracted, diminishes the energy of the blast; so that it is necessary that, at the point of greatest contraction, with a view to obtain the strongest draught, the relative diameter should be so calculated as to leave nearly a half of an inch of space, for the passage of the steam between the internal moveable cone and the edge of the blast pipe.

The intensity of the draught through the fire can be weakened, therefore, either by enlarging or by contracting the orifice of the blast pipe, beyond a certain limit. I have occasionally regulated the motion of an engine by the contraction of the blast pipe, leaving at the same time the regulator wide open, because by contracting the orifice more or less, the pressure behind the piston may be varied, and so regulated as to augment or diminish the effective action of the steam on the piston. The adoption of this variable blast may also be considered as an extra security, for by keeping the internal regulating cone of the blast pipe closed, while an engine is required to remain stationary, no danger could arise from the accidental opening of the regulator. EXPLANATION OF FIGURES.-Fig. 1. Longitudinal elevation of a locomotive boiler, part of the smoke box being removed to show the extremity of the blast pipe. The circular portion of the boiler between A and L is omitted. Fig. 2. Plan of the orifice of the blast pipe, showing the regulating internal cone B, with its three guide ribs b, b, b, upon an enlarged scale; A, smoke box; B, regulating cone of the variable blast; b, b, b, three thin ribs or feathers, attached to the regulating cone B, for the purpose of keeping the cone B exactly in the centre of the blast pipe; C, vertical rod, to which is attached the regulating cone; D, part of the chimney; E, blast pipe; F, hand gear to work the cone B; K, graduated plate fixed to the fire box, to show the position of the cone B, and the exact area of the orifice of the blast pipe; L, fire box. H. H. EDWARDS. -Civil Engineer and Architect's Journal.

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We are indebed to a friend for the following clear and satisfactory expla nation of the recent rupture of the boiler of the steamer Mohegan. It will be seen that the fault was not in the form, but in the construction of the boilSuch accidents serve as useful lessons to the careful mechanic-by teaching the necessity of increasing the strength of the boiler in those portions which have hitherto been left comparatively weak.

er.

EXPLOSION OF THE BOILER OF THE STEAMBOAT MOHEGAN.-APRIL, 1843. The steamboat Mohegan, at the time of this explosion, was furnished with two copper multiflue or tubular boilers, two and a half years old, constructed after the plan of locomotive engine boilers, for railroads. The tubes, (260, or thereabouts, in number,) had a length of about eleven feet, with a diameter of about two and a half inches. One only, of the boilers burst. The rupture was in the flank of the outer cylinder or shell, partially underneath and nearly midway of the length of the tubes. It evidently commenced at an arm-hole, three by seven inches, in the side of the boiler. From this arm-hole a rent was made each way to the nearest seam or row of rivets, the portion separated hanging like a flap, of the width of a single sheet of copper, and forming an opening for the escape of hot water and steam, of about three square feet. The explosion was accompanied by a loud report, and the end of the boiler nearest the rupture, was raised some three feet from its bed. The circumstances attending this explosion do not appear to warrant the conclusion of its having been produced either by a deficiency in the supply of water, or from an undue pressure of the steam. The part which gave way, was low down in the boiler, and was not exposed either to the direct or indirect action of the heat, and could not therefore have been weakened from this cause, and had there been an extraordinary pressure of steam, the boiler, if properly constructed, would have yielded internally, as in all similar cases of locomotive boilers, by the rupture of one or more of the tubes, and the effect would also most likely have extended to the boiler, between which and the ruptured one there was a free communication.

The cause, therefore, for the bursting of the boiler was in all probability,

ture.

the want of a due degree of strength in the boiler itself, at the place of rupIt is possible that this defect may have been in the copper. The sheet which gave way might have contained, perhaps, some flaw not discovered by the manufacturer. Supposing the material to be sound in all respects, still there were, in our view, mechanical defects in the structure of the boiler itself.

1st. The edge of the arm-hole was not protected from rupture by a band soldered and riveted thereto.

2d. The boiler rivets were too large, or too near together, by which, too much of the metal or substance of the copper was cut away.

3d. There were no stay-bars or bolts to support the sides or shell of the boiler, and no bands enclosing it.

This last circumstance is, we think, very conclusive as to the want of the requisite strength in the boiler. For the distance of eleven feet, the length of the tubes, no stay-bolts were inserted. The diameter of the shell of the boiler of this portion, is about eight feet. Its form cylindrical. With so great a diameter and length, the effect of the pressure of the steam acting with all the advantage of the funicular power in forcing out the sides of the boiler, it was scarcely possible that the copper should long be able to stand.

The locomotive boilers used upon railways are unquestionably the safest form of boilers, and for the very good reason that the shell is so much stronger than the tubes, that the latter under an undue pressure are always the first to yield, and as they are of small size the rupture of one or more is not attended with serious consequences. Each tube becomes in fact, a safety valve and the steam finds vent in a mode not likely to produce serious injury. The boiler of the Mohegan had less relative strength than a locomotive boiler, inasmuch as it was constructed of copper instead of iron, and of much greater magnitude. It is true that the former was designed to be worked with a less pressure of steam, but the difference in this respect was less than the difference in the absolute strength of the two descriptions of boilers.

A most important consideration in the construction of the tubular or multiflue boiler, is the giving to the exterior cylinder or shell a greater degree of strength than is possessed by the tubes. There is no difficulty in accomplishing this, even with boilers as large as those of the Mohegan. When so constructed, they are the safest form of boiler, and as they are not surpassed for effectiveness in generating steam, we hope that the case of the Mohegan will not be quoted to their prejudice, but serve rather to induce a more full investigation and thorough appreciation of their merits.

FULTON.

REPAIRS OF RAILWAYS.

All the first attempts at railways in the United States, as their defects of construction were developed in working them, were over and above that expense, put to heavy annual expenditures in correcting those defects, which

belonging more properly to construction, should have been charged to the capital in the road; but the whole appearing in one item under the head of ordinary repairs, has led to very exaggerated impressions as to the costliness of merely maintaining a railway, and which are yet indiscriminately entertained of the old and more modern structures, without considering that in the latter most of those original defects have been corrected, and that principally by a more liberal outlay in their first construction.

A case particularly illustrating this fact, is found in the Baltimore and Ohio railroad, among the earliest of these enterprizes, which after contending for several years with every difficulty, was nigh being abandoned in despair, and was only saved by Mr. M'Lane being called to preside over it in 1837, at which period he describes its condition in the following terms.

"The main stem to Harper's Ferry was in a state of utter delapidation, the moving power and machinery inadequate to the accommodation of the business actually offered for transportation. The department of repairs of the road was both expensive and inadequate, consisting of an unnecessary amount of superintendance without the requisite skill, and under large outlays, the road was annually becoming worse. All the repairs of machinery were made by others, under contracts at high prices, and so inadequately performed, that every part of the machinery was daily becoming more unfit for use. Public confidence appeared to be entirely withdrawn."

It is from its operations under this condition of things that most of the arguments have been drawn, and are to this day appealed to against the railway system in general. The report shows that from 1837 to 1842 the average annual expenditure in remodelling the road, was for those five years, $83,400, or $850 per mile, being $370 for labor, and $480 per mile for materials; and that now, although laboring under many miles of flat bar road, the expense for labor of adjustment and repairs of road, is reduced to

$650 per mile per annum. Instead, therefore, of the above dark picture of

1837, we have now its condition in 1842, described by Mr. M'Lane in the following bright colors; and considering the disadvantages still, of this machine, in heavy grades, short curves and flat rail, its present success may be appealed to, as the complete triumph of the modern railway system in judicious hands and in suitable locations.

"Here is now a system of railroad operations reduced to a scale of greater cheapness and economy than any other known to us in Europe or the United States, and brought to its present perfection by nearly five years of arduous toil and the exercise of all the skill and science the company could employ. It is daily complying with all the demands of trade, and giving universal satisfaction to the public, with fewer interruptions, and at less cost of transportation, than any other known road."

The reports from several roads for a series of years, furnish the following rates per mile per annum, for adjustment and repair of road, but as no details are given, it is not known what proportion belongs to repairs and what to re-construction, and the average being struck on merely the distance be