If the reader's interest in any particular part of your narrative will depend upon an understanding of a bit of mechanism or a scientific theory, you must be careful to supply the information beforehand. However trite to a mechanic or a scientist the principle may be, you must not assume that the casual reader knows it. He probably does not know it, or if he does, more than likely he has forgotten it. On the other hand, you must not appear to be self-assertively instructing him. What you can do is this: you can politely seem to be recalling something to his memory, and can thus make the point clear, so that your future use of it will not fall flat. To add a semblance of reality, it will be permissible to employ a few technical terms; but these also must be indisputably clear in meaning, and their use must not be pedantic. You should study, however, to put into the mouths of your characters the vocabulary that would be actually used by the kind of people you represent. Genial humor is a fine asset to a writer of this type of narrative. If you can be artistically serious and philosophically gay at the same time you will not fail to please. The relationship of stories of scientific discovery and mechanical invention to imaginary voyages is testified to by the reader's expectation of a display of wit. But in the scientific, ridicule is softened down to genial logic. Although the aim in this kind of narrative is good construction rather than character-sketching, yet every neat touch of portraiture that you can add will help draw your composition away from the mere exercise and toward the literary production. If you should choose your theory in the realm of art, you would by that very choice raise your story above the ordinary--I mean to say, of course, you would if you knew anything about art. Mr. Alexander Wilson Drake knows a great deal about art and has given us, besides many other beautiful surprises in _Saint Nicholas_ and the _Century_, some narratives embodying exquisite theories of shadow and color. [Illustration] =The Curious Vehicle= Reprinted by permission of the Century Company. It was midnight in early December. A dense silver mist hid the sleeping city, the street-lamps gave a faint yellow glimmer through the almost impenetrable gloom, the air was like the cold breath from the dying, the fog hanging in great drops on my clothing. Stray policemen had taken refuge in sheltering doorways, and my own footsteps echoed with unfamiliar and uncanny sound down the long street--the only sound that broke the midnight stillness, save the hoarse whistles of wandering and belated ferryboats on the distant river.