THAT atmospheric conditions have an important relation to health and efficiency has been pointed out by physiologists for a considerable time, but it is only in recent years that the practical application of this knowledge has been considered by engineers, particularly in the metal-mining industry. As the more easily worked mines are becoming exhausted, it is found that mines are tending to become deeper, and, in consequence, new problems present themselves-not only in the methods of mining, but also in the control of conditions which affect the health and efficiency of the underground worker, and permit the carrying-on of deep-mining schemes. Whether this latter problem is the more important in the local mines at the present time is perhaps questionable, but it is certain that, both on humanitarian and economic grounds, it deserves serious consideration. That atmospheric conditions have an important bearing on health is generally recognized, but the effect on efficiency has not always been considered. During the recent war, however, when a maximum output per worker was required, engineers, in studying the question of efficiency, discovered that the atmospheric conditions in the workshops had an important bearing both on the health and efficiency of the worker. This fact was fully recognized by the engineers employed in designing the Ford motor-ear factory. Ventilation was the first consideration, and it might well be claimed by the author of the article in the Engineering Magazine for November, 1914, that the factory was equipped with the best-built ventilation system ever installed in any factory building. If ventilation affects the efficiency of the surface worker, it is quite certain that the underground worker is affected in a similar manner, and probably to a greater extent.