against, but those excessive strains--caused perhaps by reckless flying, or the onslaught of a gale--to which a machine may be exposed. Security lies in one direction only; and that is to give a machine such a factor of safety as may be equal to all demands--normal or abnormal. So a designer calculates, before he draws out the plan of a wing, just what pressure it may have to resist in ordinary flight; and then to this he adds such strains as might be caused by diving or by violent gusts. To be completely on the safe side, to be guarded beyond question from any excessive shock, a modern-type aeroplane will be given a factor of safety that may be ten or twelve times more than is necessary in the ordinary passage of the machine through the air; and this security lies not only in its planes, say, or its controlling gear, but in every wire and bolt and stay which goes to its construction. And, by this precaution alone, has one of the gravest risks of aviation been removed. Formerly, when men built machines, they did so by what is termed rule-of-thumb; the builder, that is to say, relied upon his own judgment and experience--small though these were--and submitted his plans to no rigorous or scientific test. And so lives were lost. But now, even when he “loops the loop,” or fights a 60-mile-an-hour gale, a pilot has confidence in the machine he flies. He knows it is strong as a whole; that it has no hidden weakness or faulty spar; that he may rely upon it implicitly, even under the acutest strain. The building of an aeroplane is a problem of great difficulty--such a problem, in fact, as no other designers of craft, either for land or sea, are called upon to face. The machine must be immensely strong, and yet it is essential that it should be light. Its planes cannot bear through the air more than a certain number of pounds per square foot; and so if it is to be a practical craft, and raise a useful load, every pound that can be saved in its construction has a definite value. It is because such work is so intricate that there is special interest in a visit to an aeroplane factory. Here, in large, well-lighted workrooms, are skilled craftsmen employed. Some of the men, now risen to be foremen of their departments, made their first acquaintance with aviation in the very early days. Then there were no factories, and an inventor, regarded invariably as a “crank,” found a mechanic who was intelligent to help him, and built his craft humbly in some outhouse or shed. Now these handy mechanics, who perhaps forsook a post in a motor works to join some scheme to build an aeroplane, find themselves in positions of trust in a large and flourishing factory--turning out aircraft for Governments and for private use, and with a staff of workers under their control.