CONVERSATION 1.

OF THE NATURE OF AIR.

FATHER- CHARLES EMMA.

FATHER. That branch of natural philosophy which is called Pneumatics, treats of the nature, weight, pressure, and spring of the air which we breathe, and of the several effects dependent upon these properties.

Charles. You told us, a few days ago, that the air, though to us invisible, is a fluid; but it surely differs very much from those fluids which

you conversed upon when treating of hydrostatics.

Father. It does so: but recollect the terms by which we defined a fluid.

Charles. You distinguished a fluid as a body, the parts of which yield to the least pressure.

Father. The air, in which we live and move, will answer to this definition. Since we are continually immersed in it, as fish are in the water, if the parts did not yield to the least force, we should be constantly reminded of its presence by the resistance made to our bodies; whereas persons unaccustomed to think on these subjects are not even aware that they are surrounded with a fluid, the weight and pressure of which, if not counterbalanced by some other