68 



REPORT OP THE SECRETARY. 



necessary to form a comfortable and salubrious atmosphere. It must 

 be evident that if in an isolated space in winter we desire to have 

 air of the same salubrity and temperature as in an open space in 

 pleasant weather in summer, we must artificially impart to this air 

 not only the same degree of heat, but also an equal amount of mois- 

 ture. 



In an atmosphere entirely devoid of moisture the human body ex- 

 hales with great energy from every pore of the skin, and especially 

 from the luugs, and all the more delicate parts of the mucous membrane. 

 Such an exhalation of moisture in the ordinary condition of the body 

 would be far too great to allow of a healthy equilibrium between the 

 natural excretion and assimilation, and it is with prevision, derived 

 from long experience, that the savage anoints his body when exposed 

 to an atmosphere rendered arid by refrigeration, to prevent excessive 

 exhalation. The tendency of vapor to exhale from the body diminishes 

 with the force of vapor already in the surrounding atmosphere, and a 

 perfect equilibrium can only take place with air entirely saturated, at 

 a temperature of 98°. But at the temperature of zero almost all the 

 moisture of the atmosphere is condensed, and hence the tendency of the 

 body in the open air in winter to give off its vapor would be excessive, 

 were it not for the condensation of the vapor immediately around the 

 body and retained in the interstices of the clothing. If, however, the 

 surrounding air, without additional moisture, be heated to 70°, all 

 obstruction to evaporation is removed, and excessive exhalation is 

 the consequence. Beside this the equalization of the temperature is 

 much more perfectly effected in case of air properly supplied with 

 moisture. 



These considerations, we think, are very much neglected in the 

 processes which are adopted for warming and ventilating in this 

 country. At least in the public buildings in the city of Washington, 

 the means of supplying an adequate amount of moisture have not 

 been provided, or, if provided, are not habitually employed. It is 

 intended, however, during next winter, to continue the observations 

 in regard to this matter, and to extend them to the principal public 

 buildings connected with the government. 



It has been mentioned in previous reports that a series of experi- 

 ments was made in regard to the physical qualities and economical 

 values of different kinds of illuminating materials for light-house pur- 

 poses. Unfortunately all the notes of the experiments which had 

 been made on this subject were destroyed in the fire, and during the 

 past year the principal portion of time which I could spare from 



