First of all in building a model locomotive, as in every other class of engineering work, it is necessary to get the measurements correct in spacing out the different parts to be joined together; and do not think that because it is only a model you are making that any off-hand way will do, because you will find before the engine is half finished that great accuracy is necessary if you wish your model to work. A slight mistake in the measurements of a large engine will cause so much friction as to take half its power to overcome. The same mistake with your model will stop it entirely. In soldering be careful to get the metal thoroughly heated. You will then get a firm joint--otherwise not. [Illustration: Fig. 1.] [Illustration: Fig. 3.] [Illustration: Fig. 4.] [Illustration: Fig. 6.] In giving these directions we assume that the boy who will undertake to follow them is accustomed to the use of tools to some extent. If not, he will have to learn as he advances by repeated experiments. Try your experiments on something else. In soldering, for instance, solder pieces of brass together until you learn to make a joint. Don't try your experiments on your model, or you will grow discouraged before you are half through. A word more about soldering. Do not touch the metal with the soldering-iron and then take it away. You might be able to solder in that way but the joint would not hold, but fall apart at the first pressure or slight blow. Soldering on the best work should be used very seldom, and all the fastenings should be either done by riveting, screwing or brazing, and it is hardly necessary to remark that no part of a boiler should be soldered which comes in contact with the flame of the lamp or furnace. Brazing had better not be attempted by any boy who has not been practically taught the art, unless it be on small joints. To braze the seams of a model boiler would require a forge fire, or a very powerful gas blast--too expensive for the amateur. Small things such as a broken slide valve, rod, etc., can be brazed by using a gas blowpipe. This will cost but little to make, and as it will be useful, we explain. See Fig. 1. To make a blowpipe such as is pictured in Fig. 1, first get a small piece of brass tube, A, of about half an inch diameter, and 5 inches long. Drill a hole at 2 inches from one end, and insert a piece of gas pipe, B, soldering it in place. Now take a glass tube a quarter of an inch in diameter and 7 inches long, hold one end in a gas flame, and when red-hot draw it out to a fine point, then file round and break off the tip, leaving a small hole. Now take a sound cork and squeeze it into the tube A as at C, drill a quarter inch hole through its center, insert the glass tube D, and the blow pipe is finished.