work of twenty-four horses, working night and day. Emma. But the horses cannot work incessantly. Father. They will work only eight hours, at the average, out of the twenty-four, therefore since the engine is kept continually at work, it will perform the business of seventy-two horses. The coals consumed by this engine, are about seven chaldrons per week, or one chaldron in twenty-four hours. By the application of different machinery to this engine, it raises the malt into the upper warehouses, and grinds it; it pumps the wort from the under-backs into the copper; raises the wort into the coolers; it fills the barrels when the beer is made; and when the barrels are full and properly bunged, they are, by the steam-engine, driven into the store-houses in the next street, a distance of more than a hundred yards, and let down into the cellar. Charles. I do not wonder then that Dr. Darwin should anticipate the still farther extension of this useful power: Soon shall thy arm, unconquer'd steam! afar Emma. Why does Dr. Darwin, in the passage you quoted the other day, call it explosive steam? Father. From a great variety of accidents, that have happened through careless people, it appears that the expansive force of steam, suddenly raised, is much stronger than even that of gunpowder. At the cannon foundry, in Moorfields, some years ago, hot metal was poured into a mould that accidently contained a small quantity of water, which was instantly converted into steam, and caused an explosion that blew the foundry to pieces. A similar accident happened at a foundry in Newcastle, which occurred from a little water having insinuated itself into a hollow brass ball. that was thrown into the melting-pot. Charles. These facts bring to my mind a circumstance that I have often heard you relate, as coming within your knowledge. Father. You do well to remind me of it. The fact is worth recording. A gentleman who was carrying on a long series of experiments, wished to ascertain the strength of a copper vessel, and gave orders to his workmen for the purpose. The vessel, however, burst unexpectedly, and in the explosion, it beat down the brick wall of the building in which it was placed, and was, by the force of the steam, carried fifteen or twenty yards from it; several of the bricks. were thrown seventy yards from the spot; a leaden pipe suspended from an adjoining building, was bent into a right angle; and several of the men were so dreadfully bruised, or scalded, that for many weeks they were unable to stir from their beds. A very intelligent person, who conducted the experiment, assured me that he had not the smallest recollection how the accident happened, or by what means he got to his bed-room after it. Emma. Is it by the force of steam that bones are dissolved in Papin's Digester, which you promised to describe ?* Father. No; that operation is performed by the great heat produced in the digester. Plate VII. Fig. 26. is a representation of one of these machines. It is a strong metal pot, at least an inch thick in every part; the top is screwed down, so that no steam can escape but through the valve v. Charles. What kind of a valve is it? Father. It is a conical piece of brass, made to fit very accurately, but easily moveable by the steam of the water when it boils: consequently in its simple state the heat of the water will never be much greater than that of boiling water in an open vessel. A steel-yard is therefore fitted to it, and by moving the weight w backwards or forwards, the steam will have a lesser or greater pressure to overcome. * See Vol. I. Of Mechanics, Conversation III. Emma. Is the heat increased by confining the steam? Father. You have seen that, in an exhausted receiver, water not near so hot as the boiling point, will have the appearance of ebullition. It is the pressure of the atmosphere that causes the heat of boiling water to be greater in an open vessel, than in one from which the air is exhausted. In a vessel exposed to condensed air, the heat required to make the water boil would be still greater. Now by confining the steam, the pressure may be increased to any given degree. If, for instance, a force equal to 14 or 15 pounds be put on the valve, the pressure upon the water will be double that produced by the atmosphere, and of course the heat of the water will be greatly increased. Charles. Is there no danger to be apprehended from the bursting of the vessel ? Father. If great care be taken not to load this valve too much, the danger is not very great. But in experiments made to ascertain the strength of any particular vessel, too great precautions cannot be taken. Under the direction of Mr. Papin, the original inventor, the bottom of a digester was torn off with a wonderful explosion: the blast of the expanded water blew all the coals out of the fire-place, the remainder of the vessel was hurled across the room, and striking the leaf of an oaken table an inch thick, broke it in pieces. The least sign of water could not be discerned, and every coal was extinguished in a moment. CONVERSATION XLI. Of the Barometer. Father. As these conversations are intended to make you familiar with all those philosophical instruments that are in common use, as well as to explain the use and structure of those devoted to the teaching of science, I shall proceed with an account of the barometer, which with the thermometer are to be found in almost every house. I will show you how the barometer is made, without any regard to the frame to which it is attached. A B (Plate VII. Fig. 27.) is a glass tube, about 33 or 34 inches long, closed at top; that is, in philosophical language hermetically sealed; D is a cup, basin, or wooden trough, partly filled with quicksilver. I fill the tube with the quicksilver, and then put my finger upon the mouth, so as to prevent any of it from run |