Father. Dr. Desaguliers, who, in the middle of the last century, entered at large into the discussion, maintains that captain Savery was wholly indebted to the marquis, and to conceal the piracy, he charges him with having purchased all the books which contained the discovery, and burned them. Captain Savery, however, declared, that he was led to the discovery by the following accident "Having drank a flask of Florence wine at a tavern, and thrown the flask on the fire, he perceived that the few drops left in it were converted into steam; this induced him to snatch it from the fire, and plunge its neck into a bason of water, which by the atmospheric pressure, was driven quickly into the bottle."

Emma. This was something like an experiment which I have often seen at the teatable. If I pour half a cup of water into the saucer, and then hold a piece of lighted paper in the cup a few seconds, and when the cup is pretty warm, plunge it with

the mouth downwards into the saucer, the water almost instantly disappears.

Father. In both cases, the principle is exactly the same: the heat of the burning paper converts the water that hung about the cup into steam, but steam being much lighter than air, expels the air from the cup, which being plunged into the water, the steam is quickly condensed, and a partial vacuum is made in the cup; consequently the pressure of the atmosphere upon the water in the saucer forces it into the cup, just in the same manner as the water follows the vacuum made in the pump.

Charles. Is steam, then, used for the purpose of making a vacuum, instead of a piston?

Father. Just so and Dr. Darwin ascribes to captain Savery the honour of being the first person who applied it to the purpose of raising water:

Nymphs! You ere while on simmering caldrons

play'd,

And call'd delighted SAVERY to your aid,
Bade round the youth EXPLOSIVE STEAN aspire

In gathering clouds, and wing'd the wave with
fire:

Bade with cold streams the quick expansion stop,
And sunk the immense of vapour to a drop.
Press'd with the pond'rous air the piston falls
Resistless sliding through its iron walls;

Quick moves the balanc'd beam, of giant birth,
Wields his large limbs, and nodding shakes the
earth.

Emma. I remember the lines very well will you describe the engine, that we may see how they apply?

Father. I shall endeavour to give you a general and correct explanation of the principle and mode of acting of one of Mr Watt's engines, without entering into the minutia of the several

parts.

A (Plate VIII. Fig. 35.) is a section the boiler, standing over a fire, about ha

full of water: B is the steam-pipe which conveys the steam from the boiler to the cylinder c, in which the piston D, made air-tight, works up and down, a and c are the steam valves, through which the steam enters into the cylinder; it is admitted through a when it is to force the piston downwards, and through c when it presses it upwards. b and d are the eduction valves, through which the steam passes from the cylinder into the condenser e, which is a separate vessel placed in a cistern of cold water, and which has a jet of cold water continually playing up in the inside of it. f is the air-pump, which extracts the air and water from the condenser. It is worked by the great beam or lever R s, and the water taken from the condenser, and thrown into the hot well g, is pumped up again by means of the pump y, and carried back into the boiler by the pipe i i. is another pump, likek wise worked by the engine itself, which

supplies the cistern, in which the condenser is fixed, with water.

Charles. Are all three pumps, as well as the piston, worked by the action of the great beam?

Father.

They are; and you see the piston-rod is fastened to the beam by inflexible bars; but that the stroke might be perpendicular, Mr. Watt invented the machinery called the parallel joint, the construction of which will be easily understood from the figure.

Emma. How are the valves opened and shut?

Father. Long levers o and p are attached to them, which are moved up and down by the piston-rod of the air-pump E F. In order to communicate a rotatory motion to any machinery by the motion of the beam, Mr. Watt, makes use of a large fly-wheel x, on the axis of which is a small concentric toothed wheel н; a simi lar toothed wheel 1 is fastened to a rod T