issuing mixture is very little above the speed of the propagation of the flame and "back fires" frequently occur, the gas has then to be turned off, lighted again and the mixture readjusted. The ordinary acetylene lighting generator supplies gas at a pressure suitable for use with this torch.

In the medium pressure torch, both the acetylene and oxygen are under an appreciable pressure, which results in a constant mixture under all conditions, the proportion being 1.00 of acetylene to 1.3 of oxygen. The mixture is regulated by holes at the inner end of the detachable tip, and is not affected by any variation at the outer orifice. The pressure of each gas is kept constant as required by adjustable reducing valves in the pipe lines. The speed of the gas is greater than in the low pressure torch and back firing is practically eliminated. If the flame goes out, it can be relighted without altering the gas adjustment.

Sources of Gas.-The source of oxygen and acetylene gas supply is a question which requires careful consideration for each individual case separately. It can either be generated at the plant, or can be shipped in steel cylinders from one of the several companies who are generating and compressing it.

The points to be considered are:

1. The transportation charges for the compressed gas and cost of the cylinders.

2. The quantity of gas used and, therefore, the number of cylinders that would be required to contain it.

3. The regularity of the consumption, depending on the regularity of the work, and also of its size.

4. The cost of generating the oxygen at the plant.

The first depends upon the locality and shipping facilities, as well as in the quantity of gas that will be used. The second also depends on the first, with the consideration that the cost of cylinders might equal or exceed the cost of the generating plant.

The third might necessitate the generation of oxygen at the plant, anyway, particularly if repairs were the principal work carried out as unless the supply of work were regular and all of small size, one could not depend on keeping a sufficient supply of oxygen on hand, and this class of work is generally required im

mediately, and cannot therefore wait for shipments. The fourth depends on the quantity of the chemical that could be bought at a time and the cost of its transportation, and also on the amount of establishment charges that would be figured in.

It will be evident that the best method for the garage repair shop is that which involves the least expenditure as the amount of

Fig. 453.-Torch for Autogenous Welding and Tips for Same at A. Preheating Torch at B. Portable Welding Outfit on Wheeled Truck Shown at Bottom of the Illustration.

work done would not be sufficient to warrant the installation of an oxygen generating plant using the usual mixture of chlorate of potash and manganese dioxide. The acetylene gas may be readily obtained through the almost universal Presto-lite service and oxygen cylinders are also easily available.

The matter of cost of welding by the oxy-acetylene process depends upon many factors, and estimates can only be given by the

repairman after he has had considerable experience with the process. Some work requires preheating and careful manipulation, some must be clamped to prevent distortion, in such cases the cost of the jig or fixture is apt to cost more than the welding.

The following table gives an approximate idea of the cost of welding under the conditions stated:

APPROXIMATE COST OF OXY-ACETYLENE WELDING

Oxygen at three cents. Acetylene at one cent per cubic foot. Labor at 30 cents per hour.

Instructions for Operating.-The following instructions are taken from the literature of the Welding Apparatus Company of

Toledo, Ohio, and while intended to apply to the "Monarch" welding outfits manufactured by this concern, the processes may be followed with almost any of the garage type welding outfits offered by reputable manufacturers.

1. See that all gas connections are tight, using soap suds to discover leaks, if any.

2. Upper side of torch is for oxygen. Acetylene connection should be made to lower side of large handle.

3. When all connected up ready to light torch, turn regulators out, so there is no tension on spring and they are closed. Then turn both tank valves open full. Open both valves on torch one full turn or more; turn on acetylene slightly and light at tip; then turn on more until blaze has left tip slightly; then turn on oxygen same way until small white inner cone is formed in blaze. If blaze pops and goes out this denotes that not enough acetylene, or too much oxygen, is being used. The flame should have a small inner cone from 4 inch to 1⁄2 inch in length with an outside flame of larger proportions.

4. Welding should be done at the end of the small white cone. 5. In case there is a faint outline of a larger cone, the acetylene tap should be closed slightly, which will bring this cone just inside the small white cone, producing a neutral flame proper for a suecessful welding. (Opening oxygen will give same result.)

6. Blow pipes are adjusted for the proper working conditions and no sharp instruments should be used to clean out the welding tips.

7. Practice should begin on lighter sections of metal and gradually work up to the welding of the heavier sections.

8. When working on heavy work, water should be provided in which to cool the welding tips. Leave the oxygen tap open to expel steam formed. Do not plunge, but dip several times to gradually direct the heat to the tip. Otherwise you may crack the welding head.

Welding Cast Iron. In welding cast iron, such as automobile cylinders and machinery parts of similar character, it is necessary to preheat the part which is to be welded to a temperature which is slightly below a dull red heat, if there are no parts that will be

Fig. 454.-Illustrating Various Steps in Autogenous Welding Operation, Such as Preparation of Work and Manipulation of Torch. AMethod of Holding Crankshaft When Welding Broken Web. BForms of Grooves for Welding Various Thicknesses of Stock. C-Showing Method of Attaching Reducing Valve to Gas Tank. D-Method of Using Welding Rod. E-Showing Path of Torch Tip.

F-Filling a Hole by the Autogenous Welding Process.

injured by such heat. This heat should be applied gradually and when the whole object has been sufficiently preheated, the welding can be done. There are two reasons for preheating. First, to save