After they are prepared in this manner they have to be reheated to a welding temperature, placed under a steam hammer and welded, this last operation requiring 30-40 minutes. (0. K. Stewart.)

(6) Although large structures of wrought iron and of mild steel commonly have their parts united by bolting or by riveting, and although much ingenuity has been expended in so arranging and proportioning riveted joints as to obtain, in the joints, the greatest percentage of the strength of the material, nevertheless, cases occur in all structures where the union of these metals by welding becomes almost a necessity, or if not a necessity, a matter of con* venience and economy. Wrought iron, in addition to its many other merits, has the merit of being, par excellence, the weldable metal; mild steel also possesses this merit, but nevertheless there is always a feeling of doubt about a weld, although welds are of necessity largely trusted. No better illustration of this can be given than that of a common chain, each one of its manifold links having a separate weld. In the early days of making suspension bridges, the links, unless the wasteful process of cutting away a large portion of the bar was resorted to, were made by forming the eyes separate from the bar, and by welding them on. To obviate these difficulties, as long ago as 1845, Howard devised a plan which, while avoiding the necessity of welding on the eyes, avoided also the waste of material before alluded to.

Within the last few years a plan of "upsetting " the ends of the bars by mechanical means, so as to obtain the requisite material for the formation of the eye, has been introduced in the United States. All hough imperfection in a weld may arise from insufficient heat, or indeed from an excess of heat, or from the application of inadequate power to bring the heated surfaces together, probably by far the greater proportion of defective welds arises from the presence of some foreign body between the surfaces to be welded, or, as it is expressively called, "dirt" of some kind or another. Attempts have been made to obviate these difficulties by employing gaseous or liquid fuels, and, in rare cases, by the removal of the oxide of the metal from the surfaces, by turning or planing. The welds that were successively employed for the welding of railway tires in the days prior to the present system of making these tires by continuous rolling, and in the hoop form, comprised an ordinary scarf weld, bird-mouth welds in both directions, single-wedge welds, and double-wedge welds. In all these cases hammering was employed; but for carriage tires, at all events, the butt-weld was used.

In the early days of wrought-iron ordnance, where it was desirable to have in one piece a wrought-iron tube, so long in relation to its diameter that it would have been difficult to have welded it up as a single coil, it was made by welding two coils together, each of about half the length of the desired tube. It need hardly be said that every one of the modes of heating employed involved that the heat should proceed from the outside inwards. There were also the chances of inadequate heat; of an excess of heat; and, as has already been stated, of the presence of " dirt " in the weld. Moreover, there was the difficulty of ascertaining to what state he heat had attained; commonly in small welds the pieces were withdrawn from the fire, with the consequent risk of picking up " dirt" during re-introduction. Among the desiderata for heating for welding purposes, the mode adopted should admit of uniform heating throughout the sectional area to be welded; it should admit of absolute regulation of the heat; it should be free from the possibility of introducing either particles of fuel or gases between the welding surfaces, and it should admit also of complete inspection during the time the heating is going on. All these desiderata are afforded by electrical heating.

As regards the power of an electric current in passing through substances to develop heat, the most familiar instance is probably that of the carbon filament of an incandescent lamp, now so largely used in theatres, in clubs, and in private houses. It is common knowledge that electrical energy is compounded of electro-motive force or potential, or preferentially "pressure," multiplied into the amount of the current, or the pressure multiplied into the quantity. Whenever this electrical energy has to pass through a conductor, the resistance of that conductor to its passage destroys a certain portion of the electrical energy, which energy so destroyed reappears in the form of heat, and must appear in the very conductor which has been the cause of the destruction of the electrical energy. Therefore the amount of heat produced must be. that which was the thermal equivalent of the electrical energy destroyed. What the temperature would be, and in most cases it was the temperature reached by the conductor, as the result of the passage of a given quantity of current, which is of importance, depends not only upon the heat-units produced, but upon other considerations.

Although electric energy is represented by the multiplication of the pressure into the current, it will be found that the heating effect of any given current is independent of the pressure, and that the heat produced is in proportion to the current employed. As regards the heating effect of any given current upon different materials, if there were an absolutely perfect conductor, which offered no resistance to the passage of an electric current, no amount of electrical energy could heat it, because no extent of conductor could destroy any part of that electrical energy. On the other hand, in the case of a material absolutely impermeable to an electrical current, it need hardly be said that no heating could result, as no current could pass. Fortunately, the materials commonly welded, iron and steel, hold a very happy position as conductors in the scale of metals for the purpose of being electrically heated. The electrical resistance of metals increases, however, with an increase in their temperature.

This question of the increase of resistance due to increase of temperature has been investigated by Dr. Hopkinson, and the results obtained have been published in the Philosophical Transactions for the year 1889. This increase of resistance to the passage of the current, as the temperature increases, is of great utility in electrical welding. Consider the two ends of bars to be welded; mere ordinary rough surfaces; the first contact is made upon numerous points, through these the current passes, and they become rapidly heated, and offer more and more resistance. As endway pressure is applied, the surfaces in contact become of larger and larger area, until all are heated up uniformly. The greater current seeks those parts which, although in contact, are at a lower temperature, and this goes on until contact and uniform temperature are obtained. Having regard to the fact that the particular form of electrical energy needed for imparting heat is that of large current and low pressure, it will be seen that there is very great difficulty, amounting almost to a commercial impossibility, of transmitting electrical energy in such a form over any but very short distances, for, unless the conductors were of enormous size, they themselves would be

Injuriously heated by the passage of the current; and, moreover, the preasure required to drive these large currents through any considerable length of conductor would be so large a percentage of the working pressure at to add very greatly to (be power required. For these reasons it is desirable that the electrical energy "hould only take the form of large cuireat and low pressure in the very neighbourhood of the welding-machine itself. In all probability the great use of electric welding-machines would be for uniting pieces of special and difficult form, and for dealing with refractory metals - refractory in the tense that they do Dot lend themselves to successful welding by any of the ordinary processes.

At the Crewe works one of tbese welding machines Is in use, shutting on eyes to screwed rods of some 7/8 in. diameter; the weld is made close np to the screw, which is in no way Injured in the process. It has also been employed to weld ends to tubes, to weld together the parts of twist drills which have been broken, and to weld materials. It is obvious that a machine that gives, the power of heating any material with an absolutely controllable heat, and that enables that heat to be inspect- .1 and to be communicated without the advent of impurities, must have many uses in the arts, in preparing p;rres for brazing, for stamping, and m a variety of other ways; in fact, there can be no doubt that the exist-ence of such a machine will itself give rise to a large number of new uses. (Sir F. Bramwell.) (c) The reason for which machines have been invented, has been for the purpose of reproduring faithfully and constantly a set of conditions necessary to obtain a certain result. When the conditions in any case are few, and the product simple, generally one design of machine will be sufficient. With an Increased Dumber of conditions, how ever, the compleiity of the apnaramany casea, subdivisions into different processes to be executed by separate machinery.

What constitutes skill in a working man, for instance, is the ability co-ordinately to reproduce a number of operations or movements; to be, in other words, a perfect machine, or to produce the tame result, even if other conditions than those previously contemplated should arise.

Electric welding apparatus.

Electric welding apparatus.

To secure uniform results in the practice of a difficult operation, there are two ways possible.

1. To employ skilled help for the complex portion of the work alone.

2. To substitute for the more complex portion of the operation, one more readily controlled.

The ordinary welding process requires the greatest skill at the hands of the blacksmith for heating the metals to the proper temperature and at the right spot, while preventing the accumulation of cinder or scale. While skill may be successful with metals of high melting points and low conductivity for heat, easily fusible metals, aud especially good conductom, baffle all attempts as long as an exterior heating source is

The electric welding process has not only made it possible that operators not particularly skilled in the art of blacksmithing can produce good substantial welds, hut has created an art equally adaptable to all metala and combinations of metals.

The following are all the metala, alloys, and combinations so far actually welded with success by the Thomson