THE USE OF WOOD FOR FUEL. oil 
Heating values of different parts of the same tree may vary con- 
siderably, because of differences in moisture content, proportion of 
bark to wood, and other factors. Tests made by the department of 
forestry of the Michigan Agricultural College gave the following 
results: 
British thermal units 
(per pound). 
; : Dry 
Position. Moisture. matter. 
Green Dried 
wood. wood. 
Per cent.| Per cent. 
eachesap eal StUMIP in<-cleaictess cess ieee es wiles sie omnia 40. 2 59. 8 5, 584. 4 9, 253. 5 
25.2 74.7 7, 258. 0 9,718.5 
44.1 55.9 5, 086. 6 9,098. 5 
36. 2 63.8 5, 888. 4 9,227.1 
36. 1 63.9 5,581.9 8,735.8 
32.8 67.2 5, 870.8 8,735.8 
30.8 69. 2 6, 099. 1 8, 313.3 
35.7 64.3 5, 817, 2 9,045.8 
In a number of species the bark has a higher heating value than 
other parts of the tree. In the Northwest, Douglas fir bark is often 
a principal source of fuel in firing donkey engines. The bark of 
shagbark hickory has a high fuel value and burns with intense heat, 
but with much crackling. In the case of many woods, such as the 
cedars, the bark has a comparatively low fuel value and leaves a large 
proportion of ash. 
Root wood is little used for fuel, mainly because of the difficulty in 
getting it and its awkward form. It is interesting to note, how- 
ever, that the roots of mesquite are capable of producing more heat 
than the average butt cut, and are commonly dug up for firewood 
where other wood is very scarce. Very often mesquite roots are so 
much more developed than the rest of the tree that the name “ under- 
ground forests” has been applied to stands of the timber in semi- 
arid regions. 
The rapidity of burning may be important where quick heating is 
desired. Asa general rule the softwoods burn more readily than the 
hardwoods, while the lighter hardwoods burn more readily than the 
heavier ones. The pines, for instance, give a quicker, hotter fire and 
are consumed in a shorter time than birch, but birch gives a more in- 
tense flame than oak. On the other hand, the oaks give a more steady 
heat. Less than 5 per cent of the wood used as fuel is consumed in 
the industries, the remainder, or more than 95 per cent, being used 
for domestic purposes, where such qualities as ease of ignition, ra- 
pidity of combustion, freedom from smoke, uniform heating, or 
quickness of burning, depending on the particular results desired, are 
more important than calorific value. A few species, such as chestnut, 
butternut, tamarack, and spruce, are in ill favor for open fires be- 
cause they throw off sparks, 
