Thus you see that it is not the different degrees of gravity, but the resistance of the air, which prevents bodies of different weight from falling with equal velocities; if the air did not bear up the feather, it would reach the ground as soon as the marble. _Caroline._ I make no doubt that it is so; and yet I do not feel quite satisfied. I wish there was any place void of air, in which the experiment could be made. _Mrs. B._ If that proof will satisfy your doubts, I can give it you. Here is a machine called an _air pump_, (fig. 2. pl. 1.) by means of which the air may be expelled from any close vessel which is placed over this opening, through which the air is pumped out. Glasses of various shapes, usually called receivers, are employed for this purpose. We shall now exhaust the air from this tall receiver which is placed over the opening, and we shall find that bodies within it, whatever their weight or size, will fall from the top to the bottom in the same space of time. _Caroline._ Oh, I shall be delighted with this experiment; what a curious machine! how can you put the two bodies of different weight within the glass, without admitting the air? _Mrs. B._ A guinea and a feather are already placed there for the purpose of the experiment: here is, you see, a contrivance to fasten them in the upper part of the glass; as soon as the air is pumped out, I shall turn this little screw, by which means the brass plates which support them will be removed, and the two bodies will fall.--Now I believe I have pretty well exhausted the air. _Caroline._ Pray let me turn the screw.--I declare, they both reached the bottom at the same instant! Did you see, Emily, the feather appeared as heavy as the guinea? _Emily._ Exactly; and fell just as quickly. How wonderful this is! what a number of entertaining experiments might be made with this machine! _Mrs. B._ No doubt there are a great many; but we shall reserve them to elucidate the subjects to which they relate: if I had not explained to you why the guinea, and the feather fell with equal velocity, you would not have been so well pleased with the experiment. _Emily._ I should have been as much surprised, but not so much interested; besides, experiments help to imprint on the memory the facts they are intended to illustrate; it will be better therefore for us to restrain our curiosity, and wait for other experiments in their proper places. _Caroline._ Pray by what means is this receiver exhausted of its air? _Mrs. B._ You must learn something of mechanics in order to understand the construction of a pump. At our next meeting, therefore, I shall endeavour to make you acquainted with the laws of motion, as an introduction to that subject. Questions 1. (Pg. 22) What are those properties of bodies called, which are not common to all? 2. (Pg. 23) Why are they so called? 3. (Pg. 23) What is the cause of weight in bodies?