26 Transactions of the Royal Canadian Institute 



separate problems. It is, however, now generally accepted that the 

 influence of impure air in badly ventilated places, is within the limits 

 of all ordinary occurence, almost negligible in comparison with the 

 consequences of the purely physical conditions. 



The examination of the various physical factors has been frequently 

 confused on account of the failure to recognize that they should be con- 

 sidered along the lines of the general problem which was stated above. 

 Until recently the problems were attacked almost invariably by con- 

 sidering the direct effects of altering successively the various conditions, 

 but nearly always without paying sufficient attention to the accompany- 

 ing or the induced compensations, some of which always took place 

 within the internal mechanism and others of which might be produced by 

 a readjustment of the external conditions. 



Let us consider some very simple examples of this prevalent confusion. 

 It is a common observation and may also be deduced from the state- 

 ments in many texts of good standing, that a rise in external temperature 

 accompanied by a decrease in relative humidity, must necessarily lead 

 to an increase in evaporation from exposed surfaces. This is not neces- 

 sarily true, even if the motion of the surrounding air is the same in both 

 cases. The failure to grasp this elementary possibility at once, is appar- 

 ently due entirely to a lack of appreciation of the fundamental quantities, 

 and also to a noticeable vagueness very frequently associated with the 

 use of the term relative humidity^. The factors of temperature and 

 humidity with their general effects, have been familiar, but they have 

 persistently been examined without proper reference to the whole of 

 the problem in which their consideration is required. 



It should have been obvious in this example that the higher temper- 

 ature could be such that the accompanying decrease of relative humidity 

 might correspond to an increase in absolute humidity or vapour pres- 

 sure, thus producing a decrease in evaporation from any moist surface 

 kept at a fairly constant temperature. There would be an increase in 

 evaporation if the relative humidity were sufhciently decreased to indicate 

 that the absolute humidity was also lowered. The question of variation 

 in comfort when there was an increase or decrease in evaporation would 

 depend on the fundamental quantity of total heat loss from the body. 

 If there is a change in the cooling due to evaporation when the surface 

 is moist there are usually changes in the quantity of heat received by 

 radiation and by direct contact with the surrounding air. Considered 

 separately the influence of the factors on comfort are generally obscure, 

 but in terms of the general problem the interpretation is clear. 



iSee A. N. Shaw, Relative Humidity, Trans. Roy. Soc. Can. Vol. if, pp, 121-127 

 (1917.) 



