_Mrs. B._ By measuring the circumference of the circle, which the end of the lever would form in one whole revolution, and comparing this with the distance from the centre of one thread of the screw, to that of its next contiguous turn; for whilst the lever travels that whole distance, the screw rises or falls only through the distance from one coil to another. _Caroline._ I think that I have sometimes seen the lever attached to the screw, and not to the nut, as it is represented in the figure. _Mrs. B._ This is frequently done, but it does not in any degree affect the power of the instrument. All machines are composed of one or more of these six mechanical powers we have examined; I have but one more remark to make to you relative to them, which is, that friction in a considerable degree diminishes their force: allowance must therefore always be made for it, in the construction of machinery. _Caroline._ By friction, do you mean one part of the machine rubbing against another part contiguous to it? _Mrs. B._ Yes; friction is the resistance which bodies meet with in rubbing against each other; there is no such thing as perfect smoothness or evenness in nature; polished metals, though they wear that appearance more than most other bodies, are far from really possessing it; and their inequalities may frequently be perceived through a good magnifying glass. When, therefore, the surfaces of the two bodies come in contact, the prominent parts of the one, will often fall into the hollow parts of the other, and occasion more or less resistance to motion. _Caroline._ But if a machine is made of polished metal, as a watch for instance, the friction must be very trifling? _Mrs. B._ In proportion as the surfaces of bodies are well polished, the friction is doubtless diminished; but it is always considerable, and it is usually computed to destroy one-third of the power of a machine. Oil or grease is used to lessen friction: it acts as a polish, by filling up the cavities of the rubbing surfaces, and thus making them slide more easily over each other. _Caroline._ Is it for this reason that wheels are greased, and the locks and hinges of doors oiled? _Mrs. B._ Yes; in these instances the contact of the rubbing surfaces is so close, and they are so constantly in use, that they require to be frequently oiled, or a considerable degree of friction is produced.