In the month of October last a steam boiler exploded in Liberty-street, Pitsburgh, Pa., and the coroner's jury in the case rendered the following verdict:" That the explosion of said boiler was caused by a defect in the angle iron of the head, which blew out, and by a scarcity of water in the boiler, thereby creating a pressure within it beyond its strength to resist." Benjamin Crawford, Esq., Supervising Inspector of Steamboats for that district, having been solicited to give his opinion on the explosion, has done so in a communication to M. De Lange, Esq., which has been sent to us, and in which we find some new and very useful information for engineers, owners of steam boilers and the public at large. He states that, in his opinion, the verdict of the jury was a correct one, and says: That there was a defect in the head that blew out, and that there was a scarcity of water in the boiler at the time of the explosion, was made clear by the testimony taken in the case. Out, how much of the disaster is due to each of these causes is more difficult to determine, or whether either was sufficient to produce it without the aid of the other. The boiler was 40 inches in diameter by 22 feet long, male in the cylindrical form, without flues or tubes. The body of the boiler was said to be of the first quality of iron, one-fourth of an inch in thickness, and the heads were made of " angle " or " gunnell iron," which was halt an inch in thickness. Stay rods, seven-eighths of an inch in thickness, were attached to'the center of the heads, anil extended back some four or five feet, and were secured to the side of the boiler. The head that blew out had one of these rods. If the angle iron in these heads had been sound, it ought to have withstood a pressure within tho boiler of 300 lbs. to the square inch without ruptnru. The testimony before the coroner's jury in regard to the defect was somewhat conflicting, from the circumstance that oxydation had taken place on the ruptured parts before the examination was m:ide, which made it difficult to distinguish between tha fresli break and the previous defects. But to allow a diminution of one-third of the strength for the defects (which I think is as much as the testimony would justify), and there was still strength sufficient to bar a pressure of 200 lbs. to the square inch, which would be more than 200 percent greater than the working pressure used on the boiler. The danger from low water arises from the sides of the boiler, below the fire lines of the furnace, becoming exposed and overheated when unprotected by the water. In order to comprehend fully the state of a boiler in this condition, we must take into consideration all the circumstances connected with it, which are 1st. Tho water in the boiler, when the steam is not passing off to the engine or otherwise, is in a quiescent statealmost as much so as if there was no fire under the boiler at all. But the moment vent is given to the steam, by starting the engine or raising the safety valve, ebullition takes place, and the water becomes more or less agitated, and often violently so, when, as a natural consequence, it would come in contact with the sides ofi the boiler far above where it was at when in a state of rest. 2d. When the water falls below the fire lines of the furnace, the sides of the boiler are liable to become highly heated, sometimes so much so that the iron loses its tenacity to such a degree (when red hot) that the pressure of the steam will force its way through it, as in the case of the steamer Hartford, a few years ago, where no damage was done except to destroy one sheet of the boiler. But a high black heat of the iron is more dangerous, should the water, by its agitation, come in contact with it, as it makes steam when it is in that condition more rapidly than if it were red hot. 3d. The amount of steam that is produced from a given quantity of waterwhich is, under the pressure of the atmosphere (fifteen pounds to the square inch), j 703 timn it volume ; that is, one gallon of water will mi';e 1,700 gallons of steam, at a density of fifteen pounds to the square inch. Of course, the higher the pressure tho steam, the less will be the volume ; as, for example, when the pressure is at sixty pounds the volume will be 470, and when at two hundred pounds the volume will be 158, and so on. 4th. The quantity of water the sides of the boiler, when overheated, will make into steam. This is a matter, notwithstanding its importance, that seems not to have been investigated to any great extent, as I have been nnable to find in any works at my command treating on steam, anything in relation to it. I have, however, lately made some experiments with a view of an approximation to the quantity of water a square foot of boiler iron, one-fourth of an inch in thickness, when brought to a high black heat, will make into steam, and I found it to be about fifty-four cubic inches. I tried it both bv measurement and by weight, and the result was about the same. The time occupied was a little over one minute, but a very large amount of it was made into steam in a very few moments. The boiler that exploded on Liberty-street was of a capacity of about 192 cubic feet, and according to the statement of some of the witnesses who examined the boiler, and who were engineers of large experience, there could not have been more than about 20 cubic feet of water in the boiler at the time of the explosion. To fill the remaining space with steam at 60 lbs. to the square inchwhich was about what was in the boiler at the time of the explosionwould require two and seven-tenths gallons of water converted into steam. There was exposed to the heat of the furnace, above the water, about 18 inches of the sides of the boiler, which would make 66 square feet of surface. This, if heated to a high black heat (and there was evidence of some portions of it having been red hot), was capable of converting fifteen and one-third gallons of water into steam; about one-third of this quantity, or five and three-tenths gallons would produce a pressure of 200 lbs. to the square inch. This shows what the heated iron in that boiler was capable of doing, providing the water came in contact with it, which it would do the moment vent was given to the steam by the raising of the safety valve, or in any other way. As the safety valve was loaded to about 60 lbs. to the square inch, and as there was about that amount of steam in the boiler immediately before the explosion took place, I have no doubt but the safety valve did commence blowing off steam, which caused the water to foam over the heated sides of the boiler, and which, in my opinion, produced the explosion. In regard to the " requisites for the prevention of similar calamities," my opinion is that a well regulated system, enforced by law, similar to the steamboat law passed by Congress in 1852, is the best means to accomplish that object. Before the enactment of that steamboat law, disasters were constantly on the increase. Scarcely a week passed that tho community were not shocked by the news of some terrible steamboat disaster on the western rivers or on the lakes. Indeed, I remember of three most direful explosions having occurred in the space of one week (the Red Stone, Glencoe, and Sahda), by which more than two hundred lives wore lost. Since the law has been in force, few explosions, comparatively speaking, have occurred. This law requires all steamers carrying passengers to be inspected at least once a year, subjecting their boilers to a hydrostatic test. Tho pressure of steam is limited in accordance with the thickness of the iron, the diameter of the boiler, and the hydrostatic test applied ; and other precautionary measures are required, calculated to guard against danger from explosion. AH engineers are required to be examined, licensed and classified by the Inspectors, before they can serve in that capacity on any such steamer. Licenses are revoked for negligence, misconduct, or inattention to their duties. There arc other regulations to guard against fire, collisions, and other dangers, and also requiring all pilots to be licensed, &c. As an example of the working of this law, it may not be out of place for me to make a statement of the operations under it in this supervising district (which embraces Pittsburgh, Wheeling and Cincinnati), for the year ending on the 30th o. September. There were 205 steamboats inspected ; 1,024 licenses granted to engineers and pilots ; 30 revocations and refusals of licenses (the greater part on account of intemperate habits); 12 cases of violation of the law reported to the United States District Attorney for prosecution (principally steamboats ; one, however, was against a person for concealing gunpowder in a box and hipping it as hardware) ; 11 boilers were found defective upon inspection, some of which were repaired, and others condemned from further use. No explosion has occurred in this district, nor to any boat inspected in this district, nor has any accident of any kind happened to any inspected steamer in this district, by which life has been lost or personal injury sustained. I have no doubt but a law can be framed, which, if properly enforced, will give almost perfect security against the explosion of stationary boilers, and at the same time will not in the least operate against the interests of proprietors.