M A C M A 157 which lias given much impetus to these is the now not unfrequent necessity for turning or shaping steel in a more or less hard con dition, for doing which these and natural grinding-stones are the only substances practically available on a large scale, while the rapid wear of the latter unfits them for many of the purposes to which the artificial preparations of emery can be applied with great advantage. Accordingly emery wheels are now mounted for use in a great many different ways, either on slide-rests as turning tools, in emery planers and e mcry sh ap ing mach ines (such as fig. 15), and various others in which they take the place of steel cutters, or as tool grinders either general or special, in which the rival material, so far from supplanting steel, does much to wards increasing its efficiency, by enabling the process of grinding to be applied to many cutting tools which could previously be sharpened only with much greater labour and cost by other methods. ,, . . Saws, grooved rimers FlG - l.-Einery Shaping Machine. and screw taps, and twist drills are familiar instances of this appli cation. A high rate of speed is essential for obtaining the full effect of an emery wheel, half a mile a minute being by no means an unusual or excessive rate of travel for its cutting surface. A con siderable amount of heat is consequently developed at the point, of contact with the work, and the composition of the wheel must be such that it can endure this without injury. Some which could not fulfil this requirement have long been used by native workmen in India, but others which could fulfil it were patented in England in 1842, though for years after this they were but little known or used. Punching and shear ing machinery holds the same isolated position amongst machine tools that punches and shears occupy amongst cutting tools used by hand if indeed either the one or the other can be regarded as cutting tools at all. Yet, for performing.rapidly and in many cases without any waste of material, shears can often claim superiority to any other means available for ac complishing the same ends. The diagram (fig. 16) shows the old arrangement known as cropping shears, still in use at many iron-works, where early appliances seem to enjoy a remarkable vitality. An ex ample of a self-contained shearing and punching machine is given in fig. 17. The apparent ease with which machines of this kind, acting with a slow quiet stroke, shear or perforate plates of iron, even when of considerable thickness, gives an altogether false impression of the amount of power which the operation requires. Arrangements for obviating the difficulty of placing the work exactly in the correct position for each one of a series of holes to be punched in it were devised by Maudslay ; his plan, which is the one now usually adopted, being to place a traversing table in front of the machine, from some part of which it is moved to a distance depending on the "pitch" of the holes after each stroke of the punch. Another system, by which the holes could be arranged in any required pattern, was subsequently invented by Roberts. The above examples of workshop tools have been confined to those to which the requisite power is transmitted from an inde pendent steam-engine or some other prime mover the usual mode of transmission being by lines of shafting carrying pulleys or drums. Belts pass from these to similar pulleys, which may be observed on many of the machines in the engravings. But this is not the invariable method. The prime mover may itself form part of the machine, as it docs in the case of a steam-hammer. Or steam may be dispensed with and water confined under a high pressure substi tuted, which constitutes ia hydraulic system of distribution, now largely applied to the working of cranes and many other purposes, and to some extent also to machine tools. Punches and shears lend themselves readily to this system on account of their slow move ments ; so, too, do riveting machines. The distribution of power by hydraulic means, and also by compressed air, was patented by Mr Bramah in 1796. Another formidable rival to steam also has FIG. 16. Cropping Shears. now sprung up in the shape of electricity, and the results from it which are promised to us and which indeed seem likely to be ob tained will go far towards revolutionizing all our present ideas as to the difficulty of transmitting power to a distance, and will work a complete transformation in the aspect of the machine tools of the future. FIG. 17. Punching and Shearing Machine. One other class of machines must be mentioned before conclud ing, viz., measuring machines. The greatly increased accuracy of modern work has rendered necessary the recording of very minute dimensions, such as are quite beyond the measuring powers of ordi nary rules and callipers. Difference engines, i.e., machines which can measure minute differences between two articles such as a standard gauge and an intended copy of it have thus found a place in engineers works. To their arrangement and manufacture, as well as to that of standard measuring bars and gauges, Sir J. Whitworth has paid great attention, and he has achieved such success that in his workshop measuring machines a difference of TffWiF P ar t f an i ncn i g readily appreciable. At the standards de partment of the Board of Trade there is one of these machines^ used for the verification of standard gauges, which reads to the TTTTnnnr f an i ncn > an d with his most sensitive machines which, however, require great care and special precautions in their use the Tinreinnr part of an inch can be detected. The social influence of machine tools we cannot discuss, though it is a subject upon which the diffusion of correct ideas is greatly needed. The days of mill-burning and implement-breaking mobs indeed are past, but the effect of the introduction of the machinery of which these tools are the parents is one which is still much mis understood. More particularly is this the case amongst the hand- working classes in England, who see clearly the local and temporary hardships which its introduction occasionally causes, but are blind to the greatly preponderating advantages which they reap from it in an especial degree. (C. P. B. S.) MACKENZIE, SIR ALEXANDER (1755-1820), a Canadian explorer, was a native of Inverness. Having emigrated at an early age to Canada, he was for a number of years engaged in the fur trade at Fort Chipewyan, on the north side of the Lake of the Hills, and it was there that his schemes of travel were formed. His first journey (3d July to 27th September 1789) for which he had prepared himself by a year s study in England of astronomy and navigation was from Fort Chipewyan along the Great Slave Lake and down the river which now bears his name to the Frozen Ocean ; and his second (October 1792 to July 1793) from Fort Chipewyan up the Peace river across to the Columbia river, and thence westward to the coast of the Pacific at Cape Menzies, opposite Queen Charlotte Islands. The narrative of these expeditions (Voyages through North America to the Frozen and Pacific Oceans, London, 1801) is of considerable interest from the information it contains about the native tribes ; and it is prefaced by an historical dissertation on the Canadian fur trade. Mackenzie was rewarded for his discoveries by the
honour of knighthood in 1801.