652 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
the ventilation of the lungs, call the “ tidal air.” In the additional 
quantity which is drawn in by the express trains, we have the “ com- 
plimental air,” and in the excess which is forced out by express trains 
‘the “ reserve or supplemental air.” 
These three, the tidal, complmental, and supplemental, we may 
term the “ respiratory or ventilating capacity ” of the subway. 
Finally, there is an amount of air which remains in the subway and 
is not immediately forced into the streets by any combination of local 
and express trains; this we may call the “ residual air.” 
This terminology is appropriate and convenient for general pur- 
poses, and it is suggested that it should come into use among ventilat- 
ing and sanitary experts in dealing with ventilation problems of 
much less strictly physiological character than those to which it has 
hitherto been confined. 
TEMPERATURE AND HUMIDITY. 
From an early period in the construction of the road, an effort had 
been made to observe the temperature and humidity at a number of 
points by means of automatic, recording thermometers. Later, when 
the sanitary conditions were being made the subject of investigation, 
these records were critically examined and the observations put upon 
a more exact basis. 
Throughout the six months’ investigation with which this paper is 
concerned the subway was generally warmer than the streets. The 
only exceptions were when the outside temperature rose rapidly after 
a prolonged low period. This usually occurred in summer in the 
middle of the day, and in winter after a cold snap. 
The excess of subway temperature over outside temperature in- 
creased considerably during the autumn and winter months. In the 
early part of July the difference between the temperature for the 
whole day inside and outside of the subway was less than 5°. In the 
latter part of September it was over 10°. In January it was at some 
stations about 20°. An average daily difference for a week of 30° 
was found at one station. 
The subway grew warmer as the summer advanced. It averaged 
81° through July, 1905. In the week of August 4 to 10 it was 83.4°. 
Thereafter it declined very gradually, until the latter part of Septem- 
ber, when it was 76°. 
In the week of September 29 to October 5 there was a slight rise to 
78°, corresponding with a rise of temperature out of doors. This was 
followed by a more rapid decline than had occurred at any time be- 
fore. Uncomfortably high temperatures were not again experienced 
in 1905. 
During its hottest period the temperature of the subway followed 
the temperature of the outside air, except in the more extreme and 
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