932 JOURNAL OF FORKSTKV 



and Pomes rosetcs are the basement rotting fungi. These are occa- 

 sionally found above the basement in new timber which has been re- 

 cently brought in from the lumber yard. Most of these fungi are killed 

 at moderately high temperatures; therefore they may be destroyed in a 

 new building by heating it several times for a day or two to 115° F. 

 This treatment will have no value with the roof-rotting fungi which 

 thrive at higher temperatures. 



"A moderate amount of heat, however, if continuously applied is of 

 value in preventing the growth of any variety of fungus by keeping 

 the lumber well below the dew point, so that it cannot absorb enough 

 water from the air for the requirements of the fungus. This result 

 can be obtained in roofs by increasing the heat insulation, so that the 

 escape of heat is sufficiently retarded to keep the temperature of the 

 wood below the dew point. A double roof is advisable to prevent the 

 planks from rotting in the center, the outer planking being thoroughly 

 creosoted and separated from the inner by mopped, tarred paper. 



"An air space has been frequently used to increase the heat insula- 

 tion. This is worse than useless for preventing rot in highly humidi- 

 fied buildings, as the moisture passes through the wood and condenses 

 in the cooler air space, causing rapid rotting. 



"The escape of heat through the glass windows of saw-tooth and 

 monitor roofs is an important factor, the rotting frequently being lim- 

 ited to the neighborhood of windows or skylights, where the tempera- 

 ture is slightly reduced and the relative humidity increased to the dew 

 point. The cure in this case is to put sufficient additional heating pipes 

 near the windows to compensate for the heat lost through the glass. 



"Relative humidity is the basis of all timber grading. If this fact be 

 accepted by lumber dealers, it will place timber on the same exact sci- 

 entific basis as steel or concrete. Timber must be graded according to 

 the uses to which it is to be put ; where it is exposed to humid atmos- 

 pheres it must be the best that our forests produce, and in other more 

 normal conditions nearly any sort of wood may last forever. 



"The distinction between relative humidity and absolute humidity 

 should be clearly understood. When the term humid or humidity is 

 used in every-day talk, relative humidity is meant, not absolute. Rela- 

 tive humidity is the percentage of complete saturation that the air hap- 

 pens to contain at the given time and changes with the temperature, 

 while the absolute quantity of water per cubic foot of air remains un- 

 changed. The presence of a cold-water pipe may increase the relative 

 humidity of the air near it simply by lowering its temperature, while 

 the number of grains of water in each cubic foot of air remains almost 



