4 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTURE. 
14 pounds, if the total pressure were atmospheric, would be made up 
of air. In order to have no air present and the space still satu- 
rated at 101° F., the total pressure must be reduced to 1 pound by a 
vacuum pump. Fifty per cent relative humidity, therefore, signifies 
that only half the amount of vapor required to saturate the space at 
the given temperature is present. Thus at 212° F. temperature the 
vapor pressure would only be 7 \ pounds (vacuum of 15 inches gauge) . 
If the total pressure were atmospheric, then the additional 7-J pounds 
is simply air. " Live steam " is simply saturated water vapor at a 
pressure usually above atmospheric. We may just as truly have live 
steam at pressures less than atmospheric, at a vacuum of 28 inches for 
instance. Only in the latter case its temperature would be lower, 
viz, 101° F. Superheated steam is nothing more than water vapor 
at a relative humidity less than saturation, but is usually considered 
at pressures above atmospheric, and in the absence of air. The 
atmosphere at, say, 50 per cent relative humidity really contains 
superheated steam or vapor, the only difference being that it is at 
a lower pressure and temperature than we are accustomed to think 
of in speaking of superheated steam, and it has air mixed with it to 
make up the deficiency in pressure below the atmosphere. 
Two things should now be clear : That evaporation is produced by 
heat and that the presence or absence of air does not influence the 
amount of evaporation. It does, however, influence the rate of 
evaporation, which is retarded by the presence of air. The main 
things influencing evaporation are, first, the quantity of heat sup- 
plied and, second, the relative humidity of the immediately sur- 
rounding space. 
IMPORTANCE OF CIRCULATION. 
A piece of wood may be heated in three ways — (1) by convection 
of the air and vapor or other gases, (2) by conduction through some 
body in contact therewith, and (3) by radiation. Of these three 
ways, only the first is ordinarily available for use in heating a pile 
of lumber, since by either of the other two methods only the outside 
surface of the pile could be heated; hence the necessity of a large 
and thorough circulation of air. Drjdng in a vacuum would be 
feasible if there were some means of conveying the heat to the wood. 
A single stick can be readily dried in a vacuum, as it can receive 
heat on all sides by radiation from the walls of a steam- jacketed 
cylinder; but this is impracticable when it comes to an}^ quantity of 
lumber, except in the case of superheated vapor alone, as will be 
shown later, since only the outer surface or the outside boards would 
receive the heat in this way and the inside ones would not dry. 
Even an approach to a perfect vacuum, however, is not reached in 
commercial apparatus. Moreover, the heat convection in a vacuum 
