Father. A wet finger pressed round the ge of a thin drinking-glass will produce = key: if the glass be struck so as to proace its pitch, and an unison to that pitch strongly excited on a violoncello, the ass will be set in motion, and if near the of the table, will be liable to be sha

ge

en off.

CONVERSATION XXXVIII,

Of the Winds.

FATHER. You know, my children, what the wind is.

Charles. You told us, a few days ago, that you should prove it was only the air in motion.

Father. I can show you in miniature, that air in motion will produce effects similar to those produced by a violent wind.

I place this little mill under the receiver. of the air-pump in such a manner, that the air when re-entering may catch the vanes. I will exhaust the air;-now observe what happens when the stop-cock is opened.

Emma. The vanes turn round with an ncredible velocity; much swifter than ever saw the vanes of a real wind-mill. But vhat puts the air in motion, so as to cause he wind?

Father. There are, probably, many conpiring causes to produce the effect. The rincipal one seems to be heat communicaed by the sun.

Charles. Does heat produce wind?

Father. Heat, you know, expands all odies, consequently it rarefies the air, and nakes it lighter. But you have seen that he lighter fluids ascend, and thereby leave partial vacuum, towards which the surounding heavier air presses, with a greater or less motion, according to the degree of arefaction or of heat which produces it. The air of this room, by means of the ire, is much warmer than that in the pasage.

Emma. Has that in the passage a tenency into the parlour?

Father. Take this lighted wax taper, and

hold it at the bottom of the door.

Emma..

The wind blows the flame vi

olently into the room.

Father. Hold it now at the top of the

door.

1

Charles. The flame rushes outwards there.

Father. This simple experiment deserves your attention. The heat of the room ra refies the air, and the lighter particles ascending, a partial vacuum is made at the lower part of the room; to supply the defi ciency, the dense outward air rushes in, while the lighter particles, as they ascend, produce a current at the top of the door out of the room. If you hold the taper about the middle space between the bottom and top, you will find a part in which the flame is perfectly still, having no tendency either inwards or outwards.

The smoke-jack, so common in the chim neys of large kitchens, consists of a set of vanes, something like those of a wind-mill

or ventilator, fixed to wheel-work, which are put in motion by the current of air up the chimney, produced by the heat of the fire, and of course the force of the jack depends on the strength of the fire, and not upon the quantity of smoke, as the name of the machine would lead you to suppose.

Emma. Would you define the wind as a current of air?

Father. That is a very proper definition: and its direction is denominated from that quarter from which it blows.

Charles. When the wind blows from the north or south, do you say it is in the former case a north-wind, and in the latter a south-wind?

Father. We do. The winds are generally considered as of three kinds, independently of the names which they take from the points of the compass from which they blow. These are the constant, or those which always blow in the same direction: the periodical, or those which blow six months in one direction, and six in a contrary direc-.