Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 5

_Caroline._ It has just occurred to me that there are some bodies which do not gravitate towards the earth. Smoke and steam, for instance, rise instead of falling.

_Mrs. B._ It is still gravity which produces their ascent; at least, were that power destroyed, these bodies would not rise.

_Caroline._ I shall be out of conceit with gravity, if it is so inconsistent in its operations.

_Mrs. B._ There is no difficulty in reconciling this apparent inconsistency of effect. The air near the earth is heavier than smoke, steam, or other vapours; it consequently not only supports these light bodies, but forces them to rise, till they reach a part of the atmosphere, the weight of which is not greater than their own, and then they remain stationary. Look at this bason of water; why does the piece of paper which I throw into it float on the surface?

_Emily._ Because, being lighter than the water, it is supported by it.

_Mrs. B._ And now that I pour more water into the bason, why does the paper rise?

_Emily._ The water being heavier than the paper, gets beneath it, and obliges it to rise.

_Mrs. B._ In a similar manner are smoke and vapour forced upwards by the air; but these bodies do not, like the paper, ascend to the surface of the fluid, because, as we observed before, the air being less dense, and consequently lighter as it is more distant from the earth, vapours rise only till they attain a region of air of their own density. Smoke, indeed ascends but a very little way; it consists of minute particles of fuel, carried up by a current of heated air, from the fire below: heat, you recollect, expands all bodies; it consequently rarefies air, and renders it lighter than the colder air of the atmosphere; the heated air from the fire carries up with it vapour and small particles of the combustible materials which are burning in the fire. When this current of hot air is cooled by mixing with the atmosphere, the minute particles of coal, or other combustible, fall; it is this which produces the small black flakes which render the air, and every thing in contact with it, in London, so dirty.

_Caroline._ You must, however, allow me to make one more objection to the universal gravity of bodies; which is the ascent of air balloons, the materials of which are undoubtedly heavier than air: how, therefore, can they be supported by it?

_Mrs. B._ I admit that the materials of which balloons are made are heavier than the air; but the air with which they are filled is an elastic fluid, of a different nature from atmospheric air, and considerably lighter; so that on the whole the balloon is lighter than the air which it displaces, and consequently will rise, on the same principle as smoke and vapour. Now, Emily, let me hear if you can explain how the gravity of bodies is modified by the effect of the air?

_Emily._ The air forces bodies which are lighter than itself to ascend; those that are of an equal weight will remain stationary in it; and those that are heavier will descend through it: but the air will have some effect on these last; for if they are not much heavier, they will with difficulty overcome the resistance they meet with in pa.s.sing through it, they will be borne up by it, and their fall will be more or less r.e.t.a.r.ded.

_Mrs. B._ Very well. Observe how slowly this light feather falls to the ground, while a heavier body, like this marble, overcomes the resistance which the air makes to its descent much more easily, and its fall is proportionally more rapid. I now throw a pebble into this tub of water; it does not reach the bottom near so soon as if there were no water in the tub, because it meets with resistance from the water.

Suppose that we could empty the tub, not only of water, but of air also, the pebble would then fall quicker still, as it would in that case meet with no resistance at all to counteract its gravity.

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. Gla.s.ses 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 s.p.a.ce 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 gla.s.s, 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 gla.s.s; as soon as the air is pumped out, I shall turn this little screw, by which means the bra.s.s 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 ill.u.s.trate; 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?

4. (Pg. 23) What is the reason that all bodies near to the surface of the earth, are drawn towards it?

5. (Pg. 24) If attraction is the cause of weight, could you suppose it possible for a body to possess the former and not the latter property?

6. (Pg. 24) When a stone falls to the ground, in which of the two bodies does the power of attraction exist?

7. (Pg. 24) If the attraction be mutual, why does not the earth approach the stone, as much as the stone approaches the earth?

8. (Pg. 24) If attraction be in proportion to the ma.s.s, why does not a hill, draw towards itself, a house placed near it?

9. (Pg. 25) How can the attraction of a mountain be rendered sensible?

10. (Pg. 25) Why cannot two lines which are perpendicular to the surface of the earth be parallel to each other?

11. (Pg. 26) Draw a small figure of the earth to exemplify this, as in fig. 1. plate 1.

12. (Pg. 27) If bodies were not resisted by the air, those which are light, would fall as quickly as those which are heavy, how can you account for this?

13. (Pg. 27) What then is the reason that a book, and a sheet of paper, let fall from the same height, will not reach the ground in the same time?

14. (Pg. 28) What then will be the effect of increasing the surface of a body?

15. (Pg. 28) What could you do to a sheet of paper, to make it fall quickly, and why?

16. (Pg. 28) Inform me how a very dense body may be made to float in the air?

17. (Pg. 28) The air is a real body, why does it not fall to the ground?

18. (Pg. 29) The air is more dense near the surface of the earth, and decreases in density as you ascend, how is this accounted for, and to what is it compared?

19. (Pg. 29) What is it which causes the particles of air to recede from each other, and seems to destroy their mutual attraction?

20. (Pg. 29) Smoke and vapour ascend in the atmosphere, how can you reconcile this with gravitation?

21. (Pg. 30) How would you ill.u.s.trate this by the floating of a piece of paper on water?

22. (Pg. 30) Does smoke rise to a great height in the air, and if not, what prevents its so doing?

23. (Pg. 30) What limits the height to which vapours rise?

24. (Pg. 30) Of what does smoke consist?