369 OF STEAM-BOILERS. 207

SOTROS

was thrown off,

the head b, ad first had deflected it outwards; this is shown by the indentation, bc, in the figure. This head was forced off all around in the line of rivets which attached the head to the boiler, the metal remaining between the rivets being less than the space occupied by them. The convex surface and the other head were thrown likewise against the furnace, and the. head indented at de, overturning the furnace and carrying it 4 feet, as already stated. The boiler finally struck against the side of the bank of earth. The piston of the weighing-machine was somewhat bent in the experiment.

struck again on furnace,

The circumstances of this experiment show that the steam rose quite gradually on account of leaks in the boiler, increasing, probably, more rapidly as the quantity of water diminished, the intensity of the fire meanwhile increasing. That at a certain period the tension within had attained about 11 atmospheres, when the boiler exploded vioLently.

The accompanying figure will serve to give an accurate idea of the appearance of this boiler after its rupture.

The cylinder of copper, before referred to, was next put in the place of the iron boiler, and the fire again kindled; the general arrangements being as before described. This boiler being longer than the former, would

I

not descend so far into the furnace, and attempt to raise the steam sufficiently high to burst it failed: there was a considerable leak, in the junction of the curved surface with one of the ends. When the water was nearly; exhausted, the fire having passed its period. of greatest heat, the cylinder was removed. and water again introduced, filling about, three-fourths of its capacity. A new furnace was constructed of stones, allowing the boiler, to rest more closely upon the fuel, and afford ing a screen from the wind, which was blow ing quite strongly. The part of the boiler in, which the leak had been observed was turned downwards, but a similar escape was found for the steam in the part now uppermost. The tension of the steam appeared to increase very slowly, and the fire passed its best ac. tion without effect; it was renewed, and as, the water became lower the tension of thes steam increased considerably. As before, nothing remarkable occurred previous to the instant of explosion, and the members of the Committee employed in the experiments were engaged in observing the boiler at the in stant it exploded. A dense cloud of smoke and flame, capped by steam, rose from the pit; the stones and combustibles were widely scattered, and the boiler was thrown, in a single mass, about 15 feet from the furnace. The noise attending this explosion was like that from the firing of an 8-inch mortar.

The boiler was rent as shown in the accomp panying figure, giving way in an irregular

line just above the probable water line on one side of the boiler, but not conforming to it. d and b were the lowest points in the two heads before the explosion. The sheet of copper was torn from the heads, unrolled and irregularly bent, adhering to the heads for only a short distance near the top of each; and the heads were bent outwards. The thickness of the copper along the line of rup

ture varies from 025 to 035 of an inch, and the metal appears to have been highly heated at one end of the torn portion. The piston of the spring-gauge was bent, the screw which attached it to the boiler broken, and the whole instrument otherwise injured; it appeared that the wire intended to draw the boiler off the furnace had slipped and impeded the action of the piston, so that no re

gister of the amount of force producing this explosion was obtained.*

The circumstances, as before, show that the steam was allowed to rise gradually until the boiler gave way. It is possible that there may be a relation between the space occupied by the water and that in which the steam is formed most favourable to the production of steam, and that when this was attained a rapid rise in elasticity took place; but there were no circumstances observed which would confirm such a view-and if it were correct, it would only affect the conclusion as far as the increase of tension might have been rapid from such a cause.

As in the former case the marks of the sediments remained in the boiler, and indicated that the water was about an inch deep when the cylinder exploded. Much more steam was formed, and more water left than in the first experiment.

These experiments, together with the one' referred to in a subsequent part of this Report, are direct and conclusive; they show that all the circumstances attending the most violent explosions may occur without a sudden' increase of pressure within a boiler. There can be no doubt, however, that if particular portions of a boiler are much weaker than other parts, they may give way in time to prevent such a catastrophe.

IX. To repeat Perkins's experiments, and ascertain whether the repulsion stated by him to exist between the particles of intensely. heated iron and water be general; and to measure, if possible, the extent of this repulsion, with a view to determine the influence which it may have on safety-valves.

The first trials upon this subject were made under atmospheric pressure. An iron bowl, about one-sixteenth of an inch thick, and having the bottom perforated with small holes, was heated to redness over a charcoal fire, and water poured into it; the mass of metal being small, was cooled down very rapidly to a temperature below redness, and the repulsion which was at first manifested between the water and iron ceased, and the water flowed rapidly through the apertures, Two thicker bowls were provided-one of wrought iron, three-eighths of an inch thick, and the other of cast-iron, seven-sixteenths of an inch thick; the bottom of each was perforated with holes, about 04 of an inch in diameter. When placed over a charcoal fire and beated to redness, water poured in só as to fill the bowls, reduced the temperature of the wrought-iron one most rapidly, but

Assuming the strength of copper at 36,000lbs. to the square inch, and that it was uninjured ly the heating, neglecting also the effect of temperature, the bursting pressure appears by calculation to have been about sixteen atmospheres. It was, no doubt, less than this,

VOL. XXV.

until the reduction was effected, the results were the same as those for the cast-iron bowl. In this latter the water rested upon the bottom without passing through the holes, either as water or as steam; steam formed slowly. and escaped from the upper surface, the whole fluid being at a temperature below the boiling point. The openings were distinctly to be seen, and appeared by measurement to have contracted about one-seventh part of. their diameter; but the repulsion was such as to render the escape of the water quite as difficult, and indeed more difficult, than that of mercury at ordinary temperatures. Removing the vessel from the fire, the water remaining in it, as the material cooled below redness, small particles of water came through at intervals; at a lower temperature large drops collected, which finally united into full stream. Some rude measurements of the quantity of water which came through when the iron was heated in water at different temperatures, showed a striking diminution at the higher temperatures. These results were^ óbviously not produced by the closing of the apertures as the bowl expanded by heat, the openings being distinctly visible at a red heat.

into a

The measurements referred to above were as follows: At 58°, 3% fluid ounces of water passed through the holes, in the cast-iron bowl above referred to, in 30 seconds; the whole quantity of water added being 4 oz.

In another experiment at 60o, 3 fluid ounces passed. Water at between 580 and 600 being thrown into a bowl previously heated to 820, 34 fluid ounces passed through; when heated to 170°, 24 ounces passed in the same time, and when heated to 660° about 21 ounces.

In another series, the same bowl being heated to redness, 4 fluid ounces of water thrown in were perfectly repelled for 15 seconds, and at the end of half a minute only three-eighths of an ounce had flowed through the openings.

Of a second quantity of 4 ounces thrown into the bowl thus cooled, 12 ounces passed in 30 seconds, next 2 ounces in the same time.

These experiments show that the amount of the force of repulsion between water and heated metal is measurable even at moderate temperatures, and rapidly increases with the increase of temperature of the metal; the temperature of the water being in each case, of the last set of experiments, nearly the same. They confirm, in this respect, the results of the vaporisation of water by metal at different temperatures.

The pressure of the column of water which was supported over the lowest of these openings at a temperature between 660° and a

P

water.

The Committee now proceed to give an account of an unsuccessful attempt to repeat Perkins's experiment referred to in the query. As it does not seem to bear upon the applica. tion of the safety-valve, they did not deem it necessary to encounter the expense of the apparatus necessary to a further trial.

The experiment of Perkins, which is more particularly referred to in the query, is that in which an opening having been made in one of his generators, containing intensely heated water in contact with red-hot metal, neither steam nor water escaped, and in which, having affixed a pipe and stop-cock to the same vessel, no steam issued through the cock when opened. To repeat this with a view to ascertain, as required in the query, the size of opening to which such a result would apply, three apertures were made,

th, th, and 4th of an inch respectively, in the sides of a wrought-iron mercury-bottle; these were closed by conical plugs connected with levers, by which the plugs could be withdrawn from the sides of the bottle. The fulerums of these levers were attached to the wrought-iron cylinder already referred to, within which, its axis coinciding with that of the cylinder, the cylindric bottle was placed. An earthenware furnace was placed below the bottle and surrounding cylinder, the latter resting upon wrought-iron bars supported by the edges of the furnace, and the former supported by a stone placed upon the grate of the furnace. Besides affording a support for the levers, the wrought-iron cylinder was introduced to protect the experimenters against injury should the bottle explode in the trials to be made with it. This apparatus having been placed in a quarry pit, adjacent to that in which the cylindric boilers were burst, water was poured into the bottle so as to fill it, the screw-plug was next passed into the neck and forced home by lateral blows from a hammer; a fire was now made in the fur. nace, and the fuel heaped up so as to fill the entire space between the mercury-bottle and wrought-iron cylinder, and to be about five inches deep above the stopper of the former. A string was attached to the lever connected with the smallest plug, and carried up the bank. The fire soon burned briskly, and it was perceived that a small quantity of steam mingled with the feeble smoke and heated air which rose above the apparatus. A bout twenty minutes after the beginning of the experiments, the leak appearing to increase, an incautious attempt was made to stop it, but without success; at this time the bottle was seen to be at a dull red heat. It was thought that but little water had been able to escape in steam through the very minute

opening which the imperfect thread of the screw gave, and it was intended to withdraw one of the plugs, when a few minutes should have elapsed to give time for the bottle to be heated to complete redness. Meanwhile a most violent explosion occurred, the body of the bottle rose in the air, the iron cylinder which served to increase the height of the furnace was thrown from its place, the earthen furnace blown to pieces, and the fire scattered far and wide through the woods. After extinguishing the fire, it was found that the iron cylinder, weighing, with the apparatus connected with it. 6131bs., had been thrown four feet from its bed; the plugs which passed into the bottle had been broken short off at the exterior of the bottle; the bottom had been forced into the ground, which was ploughed up by the fragments of the furnace, and completely wet for a con siderable distance around; one of the iron bars supporting the cylinder was thrown to a distance of 30 feet, and sunk 3 inches into the ground. The body of the bottle was found 30 yards from its position before the explosion, having penetrated 2 feet into the ground. The noise of the explosion resem bled that of a 12-pounder fully charged.

This experiment proved, first, that steam, from intensely heated water, was able to penes trate an exceedingly small opening. Although it proved nothing in regard to an aperture made in a vessel containing water only, it showed an effect produced when there was very little steam in the vessel. It verified the deduction, from theory, that but a small part of water, bighly heated, can expand into steam. if suddenly relieved from pressure. It showed that great danger must be incurred in attempts to heat water very highly, even in vessels where it has but little room to expand itself, contrary to the apinions entertained by many; and that amattempt to repeat the experiment of Perkins, unless with an apparatus capable of sustaining the most intense pressures, must be attended with great danger.

The wrought-iron bottom of this bottle was welded upon the convex surface, and a portion of the welding appeared defective, but generally it was sound. The bot om having been torn from the convex surface, or from lateral portions, the strain was a transverse one, and, from calculation, could not have been less than ten atmospheres.

X. To ascertain whelker cases may really occur when the safety-valve, loaded with a certain weight, remains stationary, while the confined steam acquires a higher elastic force than that which from calculation would appear necessary to raise the valve.