PERIODICAL LITERATURE 727 



short time and consequently the temperature is high. If combustion is 

 slow, the number of heat units per second is small and the heat gets a 

 chance to become dissipated; consequently the temperature is low. 



If a fuel is wet the water must all be evaporated during the burning 

 of the fuel, and this takes away some of the heat. To heat up a pound 

 of water from the ordinary temperature to the boiling point, evaporate 

 it, and heat the steam to the temperature of the chimney gases requires 

 about 1,220 B. T. U. Consequently for every pound of water in the 

 fuel this amount of heat goes up the chimney. This loss is present to a 

 greater or lesser extent with all fuels, but is particularly important with 

 wood. Coal may contain 2 or 3 per cent water, or 40 to 60 pounds per 

 ton. Green wood may contain 1,500 to 2,000 pounds of water per cord. 

 Air-dried hardwood holds about 720 pounds per cord. The reason for 

 demanding well-dried wood is therefore quite obvious. 



The next statements may not seem quite so evident, but they are 

 equally true. The "heat of combustion," or "calorific value." is. within 

 narrow limits, the same for all woods — that is, a pound of one wood 

 will give off almost exactly the same amount of heat as a pound of a 

 different wood. This does not mean that a cord of one wood will give 

 the same heat as a cord of any other wood, because one cord may be 

 much heavier than the other. Some woods are highly resinous — red 

 pine, for instance — and these have a slightly higher heating value on 

 this account, but the difference is not great. The reason for all woods 

 having equal calorific value, is not far to seek. Fundamentally, all 

 woods consist of the same substance, and one species differs from an- 

 other chiefly by the way this is arranged in the wood structure. Since 

 all woods do consist chiefly of the one substance, the calorific values of 

 all of them must be the same. Measurements of the calorific value 

 show that one pound of perfectly dry wood yields 8,220 B. T. U. For 

 comparison, it may be stated that one pound of good hard coal yields 

 about 12,000 to 13,000 B. T. U. and poor coals go very much lower. 

 Perhaps it would be better to compare these in terms of cords and tons. 

 One cord of air-dried maple or birch will contain al)out 3.250 pounds 

 of dry wood and about 720 pounds of moisture. Its heating value will 

 then be: 3,250 X 8.220 = 26,715,000 B. T. U., less 1,220 X 720 = 

 878,400 B. T. U., giving a net heating value of 25,836,600 B. T. I'. .\ 

 ton of coal gives a net heating value of 2,000 X 13,000 = 26,000,000 

 B. T. U. These two values are very nearly equal, so that we can say 

 that o)ic cord of ivcU-dricd hardicood (beech, birch, or niai>le) is cijiail 

 to one ton of (jood. hard coal. Other woods have heating values in 

 proportion to their weight per cubic foot. 



