LECTURES. 159 



taking in air throiigli apertures in turrets, or even by apertures ele- 

 vated as much as was found practicable in different buildings above 

 the level of the ground, great relief was often given. The wannest 

 atmosphere in sunshine was generally at the surface of the ground, 

 where no peculiar current or other s[)ecial cause gave it a dift'erent 

 position. In all cases where a ventilating power was available, the 

 simplest method of producing a cooling effect upon the body consisted 

 in inducing a current. A draught or current was agreeable or dis- 

 agreeable, dangerous or salutary, in proportion as it was adapted to 

 existing circumstances. The fan in a lady's hand and tlie punkah, 

 or large fan used in India, were very different from the ventilating 

 shaft or other instrument used to act on hundreds or thousands at the 

 same time ; they differed essentially in this, that while the former 

 merely agitated the same air again and again, changing that portion 

 in direct contact with the face or the whole of the body, the latter, in 

 producing a similar effect, entirely changed the atmosphere charged 

 with products of respiration or exhalation. 



The use of ice, however effectual in cooling air, was generally too 

 expensive. Underground channels cooled by a stream of water, 

 removed or stopped when too much moisture was communicated to 

 the air, were the most valuable and available means of reducing tem- 

 perature ; and where hot-water apparatus was provided for winter 

 use, it might often be used as a cooling apparatus in summer by run- 

 ning a stream of cold water through it. The artificial evaporation of 

 ether and water in rams could be also rendered useful in the produc- 

 tion of cold, but no such apparatus had as yet come into general use, 

 though [perfectly successful in special experiments. 



Moistening air was a comparatively simple matter, though often 

 neglected. Very pure water should be selected for this purpose, and 

 the evaporation should not be permitted under any circumstances 

 where the water was apt to be decomposed. A porcelaneous or mar- 

 ble surface was preferred for evaporation. Iron was to be avoided, 

 and steam from ordinary boilers, contaminated by oil or gases from 

 corroded metals, was not to be used. Special copper boilers, set 

 apart exclusively for this purpose, and block tin tubes, for the con- 

 veyance of the steam, were preferred in large buildings, where an 

 atmosphere had to be provided for thousands at the same period. The 

 steam prepared in this manner was also used to assist the heating ap- 

 paratus. Whenever a thermometer with a bulb moistened with water 

 indicated a difference of not more than five degrees lower than the 

 ordinary thermometer, the addition of any further increase of moisture 

 should be arrested. 



Drying air is an operation for whicli no satisfactory process has yet 

 been pointed out sufficiently economical to admit of its general prac- 

 tical application when air is warm and largely charged or saturated 

 with moisture. When the temperature is lower, and the application 

 of a slight elevation of temperature is not objectionable, the increased 

 solvent power which the air thus acquires gives it practically a drying 

 effect. In the sick chamber, in new buildings where the plaster was 

 not dry, and in all limited or confined atmospheres where it was im- 

 portant to remove moisture, nothing was more effectual than newly 



