Drums and band-wheels are contrivances by means of which motion is communicated from one abaft, revolving on its axis, to another able also to revolve in a parallel direction, and either in the in opposite directions, through the interposition of straps of leather, gutta-percha, or elastic web. The advantage of this method of trans mitting power Is, that directly the reaistance to be overcome exceeds the adhesion of the strap upon the band-wheel, the former slips, and is unable to produce motion ; whereas when teethed wheels are used, if the resistance should be suddenly increased, there is a danger of pro ducing a rupture, or at least a serious derangement, of the machinery. On the contrary, there is often a practical inconvenience in the use of straps, from the extension of the materials of which they are composed; and it is not therefore customary to resort to their use in heavy or roportant machinery. When it is desired to change the direction of motion of the shafts, the bands are crossed; and it is essential that, wherever it is possible so to do, the faces of the wheels should present slightly convex surfaces, in order to keep the bands steady. Band wheels, equally with teethed wheels, admit of an infinite variety of velocity combinations, of which the speed pulleys of the turning lathe may be cited as characteristic illustrations. Among practical men, the first motion band wheel is usually called the drum, and the second wheel is called the pulley; and the pulleys, where more than one are used, may either be fixed or loose, and made to work, by means of ratchets or of clutches, in any way that may be required.
Clutches, or the other varieties of loose coupling gearing, are used for the purpose of connecting or disconnecting parts of a machine when it may be desired to start its motion at will, or to arrest the same; whilst boxed couplings, or fixed connections, are used when the respective parts are intended to revolve continuously. The fast and loose pulley, the friction clutch, and the conical friction and reversing gear, have been introduced in order to obviate the danger which attends the abrupt connection of moving gear with portions of a machine in repose ; but in spite of the inconvenience arising from this cause, and of the danger of stripping the teeth of cog-wheels when suddenly connected, through the inertia of the materials themselves, it is almost constantly the practice to allow the follower to fall upon and to fit into, the driver by means of a series of projections and recesses.
Clutches or couplings should be placed as near to bearings or plummer blocks as possible, in order to secure the greatest possible amount of rigidity in the portions of the shafts connected.
After all, the most satisfactory way of explaining the construction of machinery is to refer to the best examples of the means of superseding the necessity for mere brute or manual labour ; and a description, such as may be found in Warr's Dynamics,' of the recently invented turning-lathes, planing, punching, boring, drilling, screwing, and wheel cutting machines, of the Nasmyth's hammer (HAMMER, POWER AND STEAM], and of the Times' printing machinery, will therefore teach the general reader far more of the power, the advantages, and the details of those various important inventions than any abstract remarks on the principles connected with them could do. The technical reader would also find it advisable to refer to special treatises on these subjects rather than to confine himself to general remarks ; and he is therefore referred, in addition to the works already mentioned, to Buchanan on Mill-work,' new edition by G. Rennie ; Coriolis, Mdcanique des eolides,' &c.; Hachette, Trait6 des Machines ;' Borgnis, Traits des Machines and the various works which treat of the, application of the power of the ordinary prime movers, such as steam, water, wind, horse, or man's power. [Movnlo POWER.] Much information on these subjects is also to be obtained from the Mechanic's Magazine, the Transactions of the Society of Arts,' the Engineer,' and from Armangaud's ' Publi cation Industrielle des Machines,' &c., &c.