that we hear St. Paul's clock so much plainer at one time than another? Father. Undoubtedly the different degrees of density in the atmospere will occasion some difference, but the principal cause depends on the quarter from which the wind blows, for as the direction of that is towards or opposite to our house, we hear the clock better or worse.

Emma. Does it not require great strength to condense air?

Father. That depends much on the size of the piston belonging to the syringe; for the force required increases in proportion to the square of the diameter of the piston.

Suppose the area of the base of the piston is one inch, and you have already forced so much air into the vessel that its density is double that

of common air, the resistance opposed 'to you will be equal to 15 pounds; but if you would have it 10 times as dense, the resistance will be equal to

150 pounds.

Charles.

That would be more

than I could manage.

Father. Well, then, you must take a syringe, the area of whose piston is only half an inch; and in that case the -resistance would be equal to only the fourth part of 150 pounds, because the square of is equal to *. ofis

Emma. You said that the air was generally the medium by which sound is conveyed to our ears; is it not always so?

Father. Air is always a good conductor of sound, but water is a still

* The square of any number being the number multiplied into itself, × 1 =

better. Two stones being struck together under water, the sound may be heard at a greater distance by an ear placed under water in the same river, than it can through the air. In calm weather, a whisper may be heard across the Thames.

The slightest scratch of a pin, at one end of a long piece of timber, may be heard by an ear applied near the other end, though it could not be heard at half the distance through the air.

The earth is not a bad conductor of sound: it is said, that, by applying the ear to the ground, the trampling of horses may be heard much sooner than it could through the medium of the air. Recourse has sometimes been had to this mode of learning the approach of a hostile army.

Take a long strip of flannel, and

in the middle tie a common poker, which answers as well as any thing, leaving the ends at liberty; these ends must be rolled round the end of the first finger of each hand, and then stopping the ears with the ends of these fingers, strike the poker, thus suspended, against any body, as the edge of a steel fender; the depth of the tone which the stroke will return is amazing; that made by the largest churchbell is not to be compared with it.-Thus it appears that flannel is an excellent conductor of sound.

CONVERSATION XI.

FATHER.

Of Sound.

We shall devote this Conversation to the consideration of some curious circumstances relating to sound; which, as depending upon the air, will come very properly under Pneumatics.

Charles. You showed us yesterday that the stroke made by the clapper of a bell was not audible, when it was under an exhausted receiver; is the air the cause of sound?

Father. Certainly in many cases