WYMAN. — COOLING OP HOSPITALS. 487 



humidity, in the volumes required for ventilation. It is a question of 

 the rate of evaporation from a perspiring surface, which again is 

 governed in great measure by the velocity of the air ; and this by the 

 improvements in the arts we can control. 



If, on the other hand, we attempt to attain our object by cooling 

 the air before it enters the ward, we are met with this fact. If air 

 absolutely humid comes in contact with warmer air also saturated, the 

 latter will be cooled, it will approach the dew-point, and, if its moisture 

 is condensed into visible vapor, will give out heat. Evaporation con- 

 sumes heat, condensation liberates heat. 



In our first experiment the previous cooling of the air did not bring 

 it to the point of condensation, but its relative humidity was increased ; 

 the rate of evaporation was therefore diminished, and to that degree 

 it was a disadvantage. 



The quantity of air required for our purpose cannot, as we have 

 already said, be determined by instruments of precision alone ; it must 

 be learned by experiment and the declared sensations of the sick. 



The movement of the air around us, and it is never still, — the 

 natural ventilation as it is called, — is much greater than is generally 

 supposed. Repeated experiments have shown that at two feet a 

 second we first feel the air as a moving body ; less than that we con- 

 sider a perfect calm. And yet at this velocity air would move from 

 end to end of our ward of 60 feet in 30 seconds, and across, it in half 

 that time, quite unnoticed by us. 



To give comfort during the excessive heats of summer the sick 

 require three or four times the air needed for satisfactory ventilation 

 in winter. It required 400,000 cubic feet an hour for our sixteen 

 patients, and yet while this large quantity was passing through the 

 ward it was only known, except at the registers, by the accompany- 

 ing sense of freshness and pleasant coolness ; it was never felt as a 

 draught. 



" The great regulator of the heat of the body is undoubtedly the 

 skin." Physiology teaches that perspiration is a secretion, in a sen- 

 sible or insensible form, constantly going on. Increased heat increases 

 perspiration, and the evaporation of this increased quantity consumes 

 in work a large portion of the heat derived from the atmosphere, and 

 thus prevents an undue rise of the temperature of the bodily organs. 

 The very intensity, therefore, of the peripheral circulation, under the 

 action of heat, leads the way to relief. 



Experiments made more than a hundred years ago prove that, if 

 the skin perspires freely and the perspiration be readily evaporated, 



