It seems that neither piety nor wit will suffice to locate post-offices or direct experiments or explain machines. Better than either of these is the ability to make the mechanism, the process, the organization transparently clear, with each bit of information given at exactly the proper moment. For, since the object of such explanation as attempts to make clear is primarily information, the main quality of the writing should be clearness. Everything that stands in the way of this quality should be made to surrender to explanation. If the subject is itself interesting or remarkable, the facts may speak for themselves, as in an account of the nebular hypothesis; if the subject is merely common, as for example the force pump, the primary aim should be clearness. Pleasing presentation, however desirable, is secondary. No amount of pleasant reading on the subject of making photographs, the working of periscopes, the organization of literary societies will be of value if at the end the reader has not a well-ordered idea of how to go to work or of how the thing of which you treat is operated.

General Cautions

For these reasons certain principles of caution can be laid down. The first caution is, do not take too much for granted on the reader's part. First of all take stock of your reader and his knowledge of the subject and then write in accordance with your discoveries. If, in explaining the bicycle to a Fiji Islander, you fail to note that the two wheels are placed tandem rather than parallel, he may form a thoroughly queer notion of the machine. And your protest, "Why, I supposed he would know that!" is in vain. This caution does not mean that you must adopt a tone of condescension, must say, "Now children," and patter on, but that you will not omit any important part of the explanation unless you are sure that your reader is acquainted with it. The second caution, which is corollary with the first, is

that you do not substitute for the gaps in the written information the silent knowledge that is in your own mind. The danger here lies in the fact that, knowing your subject well, you will write part of it and think the rest. Having for a long time practiced the high hurdles, for example, when you come to explain them you will run the paradoxical risk of being so thoroughly acquainted with the subject that you will actually omit much vital information and thus make your treatment thin. And the third caution is, avoid being over technical. An expert can always understand plain English; a layman, on the other hand, can soon become hopelessly bewildered in a sea of technicalities. Treatment of technicalities demands sense, therefore; when a term is reasonably common its presence can do no harm, but when a term is known only to the few, substitute for it, when writing for the many, plain English, or define your terms.

Centralization

Perhaps the greatest lack in expositions of this type is centralization. A reader rises from the account of a cream separator or a suspension bridge or the feudal system with the feeling that many cogs and wires and wheels and spouts and lords and vassals are involved, but without a clear correlation of all these elements into a clear and simple whole. Now a suspension bridge is much more organic than a scrap heap, and the feudal system than a city directory. It is for you as the writer to make this clear, to show that all the things are related, that they affect each other and interact. For this purpose you will find the greatest help in the device of ascertaining what the root principle is, the fundamental notion or purpose of the subject that you are explaining. For example, to make your reader see the relation of the various parts of the tachometer you should discover and present the fact that the machine relies primarily on the principle of centrifugal force as affecting the mercury that

whirls as the automobile moves. Once this principle is grasped by the reader, the various parts of the mechanism assume their proper places and relations and become clear. Now obviously this root principle is to be sought in the subject itself; here is no place for an author to let his fancy roam where it will without keeping an eye steadily upon the machine or process. You are trying to explain the machine, not some vague or fanciful idea of what the machine might be if it were like what your fancy says; therefore, in the words of the good old advice, which comes handy in most writing, "keep your eye on the object," which in this case will be the machine or the process or the organization. And the more complicated the mechanism or process, the more necessary will be the discovery of the root principle — a printing machine, for instance, with its amazing complexity, will be helped wonderfully by such a device, and the reader will welcome the device even more than he would in an explanation of how, for example, a fountain pen worksthough he will be glad for it in any case.

This root principle, nucleus, core, kernel can often be stated in one sentence. You can say, for instance, in speaking of bridges like those across the East River, "A suspension bridge consists of a roadway hung by wires from huge cables which are anchored at the ends and are looped up over one or more high supports in the stream." This sentence may not be immediately and entirely clear, but it serves to show quickly what relations parts have to each other, and to it the reader may refer in his mind when detailed treatment of the maze of wires and bolts becomes bewildering. Often this sentence need not be expressed alone; it should always be thought out in the writer's mind.

If it is expressed, such a sentence may stand at the beginning as a sort of quick picture, or it may come at the end as a collecting statement of what has preceded, or at any point where it seems to be of the most value to the reader. It may

take various forms as, for example, it may state in essence how the machine or process works, is operated, or what it is for, or of what it consists. If it occurs at the end as a summary, it may be a summary of facts in which the points made or the parts described are enumerated, or it may be a summary of essence, in which the significance or the principle of the thing is stated. In the following examples the sen tence will be found near the beginning in both cases, and in the nature of a statement of the principle of operation.

Of tools used for cutting, perhaps the most remarkable of all is the oxygen blow-pipe. This is a little tool something the shape of a pistol - which a workman can easily hold in one hand. It is connected by a flexible tube to a cylinder of compressed oxygen, and by another tube to a supply of coal-gas. Thus a jet of oxygen and a jet of coal-gas issue from the nozzle at the end of the blowpipe, and, mingling there, produce a fine point of flame burning with intense heat. If this be directed upon the edge of a thick bar or plate of steel it will in a few seconds melt a tiny groove in it, and, if the pipe be moved along, that groove can be developed into a cut and in that way very thick pieces of steel can be severed quite easily. The harder the steel, too, the more easily it is cut, for hard steel contains more carbon than soft, and that has a tendency to burn with oxygen, actually increasing the heat of the flame. A bar of iron a foot long can be cut right down the center in fifty seconds. It is said that scientific burglars have been known to use blow-pipes to open safes with; but a very strange thing about them is that, while they will cut hard steel of almost any thickness almost like butter, they are completely baffled by a thin sheet of copper. The reason of this is that copper is such a good conductor of heat that the heat of the flame is conducted quickly away, and so the part in contact with the flame never becomes hot enough to melt.1

There is another very efficient substitute for the dynamite cartridge, which may abolish blasting even in hard-rock mines. It is a hydraulic cartridge, or an apparatus that works on the principle of the hydraulic jack. Unlike dynamite, which consists of a lot of

1 Thomas W. Corbin: Engineering of To-day. By courtesy of the publishers, Seeley, Service & Co., London.

stored and highly concentrated energy that is let fly to do what destruction it may, the hydraulic cartridge is absolutely inert and devoid of potential energy when placed in the blast-hole. Only after it is in place is the energy applied to it. This it gradually accumulates until it acquires enough to burst open the rock without wasting a lot of energy in pulverizing it. The apparatus is under the direct control of the miner all the time. There is nothing haphazard about its operation.

The cartridge consists of a strong steel cylinder, made in various sizes. Disposed at right angles to the length of the cylinder are a number of pistons, or rams, that may be forced out laterally by pumping water into the cylinder. The cartridge is introduced into the blast-hole with the rams retracted. Then a quick-action pump is operated to move the rams out so that they come in contact with the rock. After this, by means of a screw-lever a powerful pressure is exerted upon the water, which forces out the rams until the rock gives way under the strain.1

Processes

The development of this kind of exposition will vary somewhat according to the nature of the subject. If you are explaining a process - how to make a campfire, or how to find the width of an unbridged river, or how to make breadyou will naturally follow the chronological order and tell what to do first, what second, and so on. If several materials are to be used in the process, you may enumerate them all at the beginning, for collection, or state them piece by piece as they are needed. For example, you may say, "In making a kite you will need so many pieces of such wood of such and such sizes, with paper or cloth, strong twine, glue, nails, etc." You may cast the whole process into a personal mood by telling how some one, perhaps yourself, did it on a previous occasion. This method, if it is judiciously used, adds interest. You must take care not to seem to encumber

1 Taken from The Century Magazine by permission of the publishers, The Century Co.