Along each side of the slip there is usually a row of tall masts with a beam projecting out sideways near the top of each, forming cranes by which the heavier parts can be hoisted into position. In other yards, again, there is a tall iron structure called a gantry along each side of the slip, while travelling cranes span across from one to the other over where the growing ship lies. These travelling cranes, worked by electricity, permit heavy weights to be handled with ease and safety. Other subsidiary cranes, meanwhile, carry the heavy hydraulic riveting machines by which riveting is done. Much riveting is done by hand, men working together in squads of four. Of these one, often quite a boy, heats the rivets in a small furnace, after which he throws them one by one to man number two, who inserts each as he receives it in its proper hole and holds it there with a big heavy hammer or else a tool called a "dolly." Number two is called the "holder-up," since he holds the rivet up in its place while the remaining two hammer it over with alternate blows of their hammers. In many cases, however, the two last described men give place to one, who is armed with a tool in shape much like a pistol and operated by compressed air obtained through a flexible tube. When he presses a trigger a little hammer inside the "pistol" gives a rapid series of blows to the rivet, completing the job more quickly than the two men can do with hand hammers. A third way of doing this operation so important in the building of a ship is by the hydraulic machine suspended from the cranes. To the casual onlooker this has the notable feature of being silent, whereas riveting by hand and still more by a pistol hammer is terribly noisy. The reason for this is that the hydraulic riveter does not hammer at all, but, like a huge mechanical hand, it takes the rivet between finger and thumb and just squeezes it down. One strange result of all this hammering in of rivets is that every ship by the time it leaves the slip has become a huge magnet, with somewhat disconcerting effects upon its own compasses, but of that more later on. Thus the great ship grows, being made piece by piece in the workshops to the shapes indicated from the mould loft and put together and riveted on the slip, until finally in due time it is ready to take its first journey. The launching of a big ship always strikes me as about the boldest and most daring thing which is ever done in the course of industry. For the huge structure, naturally top-heavy, weighing hundreds or thousands of tons, is just allowed to slide at its own sweet will. From the moment it starts until it is well in the water it is in charge of itself, so to speak, and if anything were to go wrong no power on earth could stop it once it had got a start.