From the text...'As part of a general forest-fire-research program in recent years, considerable inquiry has been made into the visibility of smoke from forest fires in an attempt to answer the questions: How far can a lookout see a smoke? What are the factors upon which this visual range depends? In what way and to what extent do these factors exert their influence? In the answer to these questions are embodied principles upon which a forest-fire-detection system should be based and upon which practical administrative action should be taken in establishing lookouts and in posting supplementary observers in time of emergency.....With this aim in mind this paper was prepared, not in any attempt to solve the whole problem of smoke visibility, but rather to treat of the theoretical aspects and to develop equations for visual smoke range to the point where experimental observations are required to evaluate constants and to determine unknown functions. The equations, if acceptable, should serve as a working hypothesis and guide in the experimentation needed for complete or practical solution of the problem.'