MRS. B. Uniformity of motion in a steam-boat is not required; but in manufactures, and in most cases in which machinery is employed, uniformity of action is essentially requisite, both in order to prevent injury to the machine, and imperfection in the work performed. A fly-wheel, which is a large heavy wheel attached to the axis of one of the principal wheels of the machinery, answers this purpose, by regulating the action of the machine; by its weight it diminishes the effect of increased action, and by its inertia it carries on the machine with uniform velocity when the power transiently slackens: thus, by either checking or impelling the action of the machine, it regulates its motion so as to render it tolerably uniform. 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 generally 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 an explanation of the motion of the heavenly bodies. 145 CONVERSATION VII, CAUSES OF THE EARTH'S ANNUAL OF THE PLANETS AND THEIR MOTION. OF THE DIURNAL MOTION OF THE EARTH AND PLANETS. CAROLINE. OH! Mrs. B., 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. I see that you are quite elated with the spirit of opposition; but tell me, what is this weighty objection? CAROLINE. You say that bodies attract in proportion to the quantity of matter they contain: now, we all know H 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 Newton. For what purpose are we endowed with reason, if we are denied the privilege of making use of it, by judging for ourselves? MRS. B. It is reasor it if which teaches us, that when we, novices in science, start objections to theories established by men of acknowledged wisdom, we should be diffident rather of our own than of their opinion. I am far from wishing to lay the least restraint on your questions. You cannot be better convinced of the truth of a system, than by finding that it resists all your attacks; but I would advise |