Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 4

_Mrs. B._ You are: you must allow, therefore, that it would be possible for attraction to exist without weight; for each of the particles of which the body was composed, would possess the power of attraction; but they could exert it only amongst themselves; the whole ma.s.s having nothing to attract, or to be attracted by, would have no weight.

_Caroline._ I am now well satisfied that weight is not essential to the existence of bodies; but what have you to object to colours, Mrs. B.; you will not, I think, deny that they really exist in the bodies themselves.

_Mrs. B._ When we come to treat of the subject of colours, I trust that I shall be able to convince you, that colours are likewise accidental qualities, quite distinct from the bodies to which they appear to belong.

_Caroline._ Oh do pray explain it to us now, I am so very curious to know how that is possible.

_Mrs. B._ Unless we proceed with some degree of order and method, you will in the end find yourself but little the wiser for all you learn.

Let us therefore go on regularly, and make ourselves well acquainted with the general properties of bodies before we proceed further.

_Emily._ To return, then, to attraction, (which appears to me by far the most interesting of them, since it belongs equally to all kinds of matter,) it must be mutual between two bodies; and if so, when a stone falls to the earth, the earth should rise part of the way to meet the stone?

_Mrs. B._ Certainly; but you must recollect that the force of attraction is proportioned to the quant.i.ty of matter which bodies contain, and if you consider the difference there is in that respect, between a stone and the earth, you will not be surprised that you do not perceive the earth rise to meet the stone; for though it is true that a mutual attraction takes place between the earth and the stone, that of the latter is so very small in comparison to that of the former, as to render its effect insensible.

_Emily._ But since attraction is proportioned to the quant.i.ty of matter which bodies contain, why do not the hills attract the houses and churches towards them?

_Caroline._ What an idea, Emily! How can the houses and churches be moved, when they are so firmly fixed in the ground!

_Mrs. B._ Emily's question is not absurd, and your answer, Caroline, is perfectly just; but can you tell us why the houses and churches are so firmly fixed in the ground?

_Caroline._ I am afraid I have answered right by mere chance; for I begin to suspect that bricklayers and carpenters could give but little stability to their buildings, without the aid of attraction.

_Mrs. B._ It is certainly the cohesive attraction between the bricks and the mortar, which enables them to build walls, and these are so strongly attracted by the earth, as to resist every other impulse; otherwise they would necessarily move towards the hills and the mountains; but the lesser force must yield to the greater. There are, however, some circ.u.mstances in which the attraction of a large body has sensibly counteracted that of the earth. If whilst standing on the declivity of a mountain, you hold a plumb-line in your hand, the weight will not fall perpendicular to the earth, but incline a little towards the mountain; and this is owing to the lateral, or sideways attraction of the mountain, interfering with the perpendicular attraction of the earth.

_Emily._ But the size of a mountain is very trifling, compared to the whole earth.

_Mrs. B._ Attraction, you must recollect, is in proportion to the quant.i.ty of matter, and although that of the mountain, is much less than that of the earth, it may yet be sufficient to act sensibly upon the plumb-line which is so near to it.

_Caroline._ Pray, Mrs. B., do the two scales of a balance hang parallel to each other?

_Mrs. B._ You mean, I suppose, in other words to inquire whether two lines which are perpendicular to the earth, are parallel to each other?

I believe I guess the reason of your question; but I wish you would endeavour to answer it without my a.s.sistance.

_Caroline._ I was thinking that such lines must both tend by gravity to the same point, the centre of the earth; now lines tending to the same point cannot be parallel, as parallel lines are always at an equal distance from each other, and would never meet.

_Mrs. B._ Very well explained; you see now the use of your knowledge of parallel lines: had you been ignorant of their properties, you could not have drawn such a conclusion. This may enable you to form an idea of the great advantage to be derived even from a slight knowledge of geometry, in the study of natural philosophy; and if after I have made you acquainted with the first elements, you should be tempted to pursue the study, I would advise you to prepare yourselves by acquiring some knowledge of geometry. This science would teach you that lines which fall perpendicular to the surface of a sphere cannot be parallel, because they would all meet, if prolonged to the centre of the sphere; while lines that fall perpendicular to a plane or flat surface, are always parallel, because if prolonged, they would never meet.

_Emily._ And yet a pair of scales, hanging perpendicular to the earth, appear parallel?

_Mrs. B._ Because the sphere is so large, and the scales consequently converge so little, that their inclination is not perceptible to our senses; if we could construct a pair of scales whose beam would extend several degrees, their convergence would be very obvious; but as this cannot be accomplished, let us draw a small figure of the earth, and then we may make a pair of scales of the proportion we please. (fig. 1.

pl. I.)

_Caroline._ This figure renders it very clear: then two bodies cannot fall to the earth in parallel lines?

_Mrs. B._ Never.

_Caroline._ The reason that a heavy body falls quicker than a light one, is, I suppose, because the earth attracts it more strongly.

_Mrs. B._ The earth, it is true, attracts a heavy body more than a light one; but that would not make the one fall quicker than the other.

_Caroline._ Yet, since it is attraction that occasions the fall of bodies, surely the more a body is attracted, the more rapidly it will fall. Besides, experience proves it to be so. Do we not every day see heavy bodies fall quickly, and light bodies slowly?

_Emily._ It strikes me, as it does Caroline, that as attraction is proportioned to the quant.i.ty of matter, the earth must necessarily attract a body which contains a great quant.i.ty more strongly, and therefore bring it to the ground sooner than one consisting of a smaller quant.i.ty.

_Mrs. B._ You must consider, that if heavy bodies are attracted more strongly than light ones, they require more attraction to make them fall. Remember that bodies have no natural tendency to fall, any more than to rise, or to move laterally, and that they will not fall unless impelled by some force; now this force must be proportioned to the quant.i.ty of matter it has to move: a body consisting of 1000 particles of matter, for instance, requires ten times as much attraction to bring it to the ground in the same s.p.a.ce of time as a body consisting of only 100 particles.

[Ill.u.s.tration: PLATE I.]

_Caroline._ I do not understand that; for it seems to me, that the heavier a body is, the move easily and readily it falls.

_Emily._ I think I now comprehend it; let me try if I can explain it to Caroline. Suppose that I draw towards me two weighty bodies, the one of 100 lbs. the other of 1000 lbs. must I not exert ten times as much strength to draw the larger one to me, in the same s.p.a.ce of time, as is required for the smaller one? And if the earth draws a body of 1000 lbs.

weight to it in the same s.p.a.ce of time that it draws a body of 100 lbs.

does it not follow that it attracts the body of 1000 lbs. weight with ten times the force that it does that of 100 lbs.?

_Caroline._ I comprehend your reasoning perfectly; but if it were so, the body of 1000 lbs. weight, and that of 100 lbs. would fall with the same rapidity; and the consequence would be, that all bodies, whether light or heavy, being at an equal distance from the ground, would fall to it in the same s.p.a.ce of time: now it is very evident that this conclusion is absurd; experience every instant contradicts it; observe how much sooner this book reaches the floor than this sheet of paper, when I let them drop together.

_Emily._ That is an objection I cannot answer. I must refer it to you, Mrs. B.

_Mrs. B._ I trust that we shall not find it insurmountable. It is true that, according to the laws of attraction, all bodies at an equal distance from the earth, should fall to it in the same s.p.a.ce of time; and this would actually take place if no obstacle intervened to impede their fall. But bodies fall through the air, and it is the resistance of the air which makes bodies of different density fall with different degrees of velocity. They must all force their way through the air, but dense heavy bodies overcome this obstacle more easily than rarer or lighter ones; because in the same s.p.a.ce they contain more gravitating particles.

The resistance which the air opposes to the fall of bodies is proportioned to their surface, not to their weight; the air being inert, cannot exert a greater force to support the weight of a cannon ball, than it does to support the weight of a ball (of the same size) made of leather; but the cannon ball will overcome this resistance more easily, and fall to the ground, consequently, quicker than the leather ball.

_Caroline._ This is very clear and solves the difficulty perfectly. The air offers the same resistance to a bit of lead and a bit of feather of the same size; yet the one seems to meet with no obstruction in its fall, whilst the other is evidently resisted and supported for some time by the air.

_Emily._ The larger the surface of a body, then, the more air it covers, and the greater is the resistance it meets with from it.

_Mrs. B._ Certainly: observe the manner in which this sheet of paper falls; it floats awhile in the air, and then gently descends to the ground. I will roll the same piece of paper up into a ball: it offers now but a small surface to the air, and encounters therefore but little resistance: see how much more rapidly it falls.

The heaviest bodies may be made to float awhile in the air, by making the extent of their surface counterbalance their weight. Here is some gold, which is one of the most dense bodies we are acquainted with; but it has been beaten into a very thin leaf, and offers so great an extent of surface in proportion to its weight, that its fall, you see, is still more r.e.t.a.r.ded by the resistance of the air, than that of the sheet of paper.

_Caroline._ That is very curious: and it is, I suppose, upon the same principle that a thin slate sinks in water more slowly than a round stone.

But, Mrs. B., if the air is a real body, is it not also subjected to the laws of gravity?

_Mrs. B._ Undoubtedly.

_Caroline._ Then why does it not, like all other bodies, fall to the ground?

_Mrs. B._ On account of its spring or elasticity. The air is an _elastic fluid_; and the peculiar property of elastic bodies is to resume, after compression, their original dimensions; and you must consider the air of which the atmosphere is composed as existing in a state of compression, for its particles being drawn towards the earth by gravity, are brought closer together than they would otherwise be, but the spring or elasticity of the air by which it endeavours to resist compression, gives it a constant tendency to expand itself, so as to resume the dimensions it would naturally have, if not under the influence of gravity. The air may therefore be said constantly to struggle with the power of gravity without being able to overcome it. Gravity thus confines the air to the regions of our globe, whilst its elasticity prevents it from falling, like other bodies, to the ground.

_Emily._ The air then is, I suppose, thicker, or I should rather say more dense, near the surface of the earth, than in the higher regions of the atmosphere; for that part of the air which is nearer the surface of the earth must be most strongly attracted.

_Mrs. B._ The diminution of the force of gravity, at so small a distance as that to which the atmosphere extends (compared with the size of the earth) is so inconsiderable as to be scarcely sensible; but the pressure of the upper parts of the atmosphere on those beneath, renders the air near the surface of the earth much more dense than in the upper regions.

The pressure of the atmosphere has been compared to that of a pile of fleeces of wool, in which the lower fleeces are pressed together by the weight of those above; these lie light and loose, in proportion as they approach the uppermost fleece, which receives no external pressure, and is confined merely by the force of its own gravity.

_Emily._ I do not understand how it is that the air can be springy or elastic, as the particles of which it is composed must, according to the general law, attract each other; yet their elasticity, must arise from a tendency to recede from each other.

_Mrs. B._ Have you forgotten what I told you respecting the effects of heat, a fluid so subtile that it readily pervades all substances, and even in solid bodies, counteracts the attraction of cohesion? In air the quant.i.ty of heat interposed is so great, as to cause its particles actually to repel each other, and it is to this that we must ascribe its elasticity; this, however, does not prevent the earth from exerting its attraction upon the individual particles of which it consists.