1 Mrs. B. Yes; in these instances the contact of the rubbing surfaces is so close, and the rubbing so continual, that notwithstanding their being polished and oiled, a considerable degree of friction is produced. There are two kinds of friction; the one occasioned by the sliding of the flat surface of a body, the other by the rolling of a circular body; the friction resulting from the first is much the most considerable, for great force is required to enable the sliding body to overcome the resistance which the asperities of the surfaces in contact oppose to its motion, and it must be either lifted over, or brake through them; whilst, in the other kind of friction, the rough parts roll over each other with comparative facility; hence it is, that wheels are often used for the sole purpose of diminishing the resistance of friction. Emily. This is one of the advantages of carriagewheels; is it not? Mrs. B. Yes; and the larger the circumference of the wheel the more readily it can overcome any considerable obstacles, such as stones, or inequalities in the road. When, in descending a steep hill, we fasten one of the wheels, we decrease the velocity of the carriage, by increasing the friction. Caroline. That is to say, by converting the rolling friction into the dragging friction. And when you had casters put to the legs of the table, in order to move it more easily, you changed the dragging into the rolling friction. Mrs. B. There is another circumstance which we have already noticed, as diminishing the motion of bodies, and which greatly affects the power of machines. This is the resistance of the medium, in which a machine is worked. All fluids, whether of the nature of air, or of water, are called mediums; and their resistance is proportioned to their density; for the more matter a body contains, the greater the resistance it will oppose to the motion of another body striking against it. Emily. It would then be much more difficult to work a machine under water than in the air? Mrs. B. Certainly, if a machine could be worked in vacuo, and without friction, it would be perfect; but this is unattainable; a considerable reduction of power must therefore be allowed for the resistance of the air. We shall here conclude our observations on the mechanical powers. At our next meeting I shall endeavour to give you an explanation of the motion of the heavenly bodies. CONVERSATION VI. CAUSES OF THE EARTH'S ANNUAL MOTION. OF THE PLANETS, AND THEIR MOTION.OF THE DIURNAL MOTION OF THE EARTH AND PLANETS. CAROLINE. I AM come to you to-day quite elated with the spirit of opposition, Mrs. B.; for I have discovered such a powerful objection to your theory of attraction, that I doubt whether even your conjuror Newton, with his magic wand of attraction, will be able to dispel it. Mrs. B. Well, my dear, pray what is this weighty objection? Caroline. You say that bodies attract in proportion to the quantity of matter they contain, now we all know the sun to be much larger than the earth: why, therefore, does it not attract the earth; you will not, I suppose, pretend to say that we are falling towards the sun? Emily. However plausible your objection appears, Caroline, I think you place too much reliance upon it: when any one has given such convincing proofs of sagacity and wisdom as Sir Isaac Newton, when we find that his opinions are universally received and adopted, is it to be expected that any objection we can advance should overturn them? Caroline. Yet I confess that I am not inclined to yield implicit faith even to opinions of the great New |