altered from the construction shown in Diagram A at 34 stations and 34 points between stations to that shown in Diagram B which illustrates the perforation made in the roof of the subway in the attempt to permit the excess heat to escape by natural means. Supplementing this change there was devised the scheme shown in Diagram C consisting of vent chambers between stations at the sides of the subway. This was an attempt to control the inflow and outflow of the subway air by the "piston-like" action of the trains and check valves or louvres hinged in a most ingenious and elaborate fashion in the side wall of the subway. As will be noted from these vent chambers ducts connected with sidewalk gratings, the idea being that upon the approach of a train the air would be compressed in the neighborhood of these chambers and the flaps would be opened and the air would be expelled through the grating as is the case, and that when the train had gone by the air suction created thereby would pull these check valves closed and air would be drawn into the subway at the next nearest hole which would probably be at the station immediately behind the oncoming train. See title page.
In theory the scheme of the louvres is most ingenious and desirable provided that the trains did in reality act as pistons within a cylinder, but such is not, and cannot be the case, inasmuch as it is clear that a local train in the compartment on the extreme right of the subway and nearest to these vent chambers would practically have to fill all four
