sured me, that he had not the smallest recollection how the accident hap pened, or by what means he got to his bed-room after the explosion.

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 111, 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 * See Val. I, of Mechanics, Conver. III.

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.

* Emma. Is the heat increased by confining the steam?

It

Father. You have seen, that, in an exhausted receiver, water not near so hot as the boiling point, will have every appearance of ebullition. 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 care be taken so as 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, the utmost precautions must be taken.

Under the direction of Mr. Papin,

the original inventor, the bottom of å 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 XIX.

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, is 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.