It is a well-known principle of modern business correspondence that if a letter refers only to one subject it is more likely to receive attention than if it deals with several; therefore if you have several different orders or directions to give it is bad policy to write them all at once, unless you are absolutely compelled to do so because they are all equally pressing. Even if there is the same degree of urgency for all, yet a practical impossibility that all should be executed at the same time, it is still the best policy to give your orders successively and not more quickly than they can be executed. The only danger of this is that the receiver of the orders may think at first that they are small matters in which postponement signifies little, as they can be executed at any time. To prevent this he should be strongly warned at first that the order will be rapidly followed by several others. If there is not the same degree of urgency for all, the best way is to make a private register of the different matters in the order of their urgency, and then to write several short notes, at intervals, one about each thing. People have such a marvellous power of misunderstanding even the very plainest directions that a business letter never _can_ be made too clear. It will, indeed, frequently happen that language itself is not clear enough for the purposes of explanation without the help of drawing, and drawing may not be clear to one who has not been educated to understand it, which compels you to have recourse to modelling. In these cases the task of the letter-writer is greatly simplified, as he has nothing to do but foresee and prevent any misunderstanding of the drawing or model. Every material thing constructed by mankind may be explained by the three kinds of mechanical drawing,--plan, section, and elevation,--but the difficulty, is that so many people are unable to understand plans and sections; they only understand elevations, and not always even these. The special incapacity to understand plans and sections is common in every rank of society, and it is not uncommon even in the practical trades. All letter-writing that refers to material construction would be immensely simplified if, by a general rule in popular and other education, every future man and woman in the country were taught enough about mechanical drawing to be able at least to _read_ it.