gas-producing capacity passing on each track every three minutes, he constructs a diagram showing how the composition of the atmosphere of the tunnel varies at successive points, and how, by an examination of the diagram thus made, it is possible to discover the maximum vitiation of the air, and consequently the extent to which the conditions are satisfied. By one or two such constructions any such problem may be solved to a degree quite within the limits of practical work, and the effect of various systems of ventilation compared. M. Godfernaux discusses various systems of ventilation, including those involving the use of shafts, fan blowers and exhausters, and air jets, and concludes with a description of the Saccardo system, in use in the Apennine tunnel of the Bologna-Pistoia line, and to the St. Gothard Tunnel. While all this investigation and discussion is of much value, it certainly seems as if the true remedy lies not so much in the removal of deleterious gases as in the absence of their production. The substitution of electric traction avoids altogether the fouling of the air of tunnels and subways, and electric locomotives are already used in the Baltimore Tunnel in the United States and elsewhere, and it seems as if this remedy is the true one to be applied in all cases. * * * * * =Liquid Air.=--The following warning appears in The Engineering and Mining Journal of March 3d: “The advertisements which are now appearing in the papers all over the country of companies which are to furnish liquid air on a large scale must be accepted with a great deal of caution. The public mind has been very adroitly worked up for the reception of these by lectures, paragraphs in the press, and other well-understood methods. Undoubtedly liquid air possesses some valuable properties, and many striking experiments can be performed with it. It is not by any means certain yet that it can be prepared, transported, and used economically on a commercial scale, or that the difficulties in the way have been overcome. We do not say that they may not be overcome in the future; but to talk, as the advertisements do, of the certainty that liquid air will soon largely replace steam in furnishing motive power is going entirely too far. Such assertions have no present basis of fact to warrant any one in making them. The liquid-air people have a great deal to do yet before they can establish their claims or carry on business on a scale that will warrant the organization of ten-million-dollar companies. The question of validity of patents is also quite an open one. It is doubtful if there is any valid patent on this subject.” * * * * *