FUMIGATION OF CITRUS TREES. 27 
figure lb. The second step in the procedure is shown in figure 2 of 
Plate VI, while the remaining steps are the same as in Plate V, 
figures 4, 5, and 6. 
In covering very large trees derricks of the nature described on 
page 21 are used. Four men are required for their manipulation, 
which is shown in Plate VII. A derrick is raised to a nearly upright 
position at each side of the tree (Plate VII, figs. 1-3), leaning at a 
slight angle backward and held in this position by the guy-rope 
attached to its top (Plate VII, fig. 3). The movable pulley of each 
derrick is then attached to a ring in the tent (Plate VII, fig. 3) and 
pulled up to the top of the derrick, where it is held (Plate VII, fig. 4). 
By pulling on the guy-rope the derrick is caused to fall forward, 
drawing the tent over the tree. 
CALCULATING THE DOSAGE. 
Having covered the trees, the next requirement is the amount of 
chemicals to use, or the dosage. The dosage is the most important 
consideration in the gas process. It varies not only with the size of 
the tree but also with the character of insect to be destroyed. Spe- 
cific recommendations of dosage for the principal insects injurious 
to citrus trees in California are given elsewhere in this bulletin 
(pp. 51-61). 
The first requirement in calculating the dosage for a tree is to 
compute the cubic contents inclosed by the tent when in position 
over the tree. Although most citrus trees possess a certain general 
similarity in shape, they are nevertheless somewhat irregular, no two 
ever being identical in all respects. This irregularity renders it 
impracticable to determine the contents to within a cubic foot or so 
of its actual volume; yet it can be approximated with a sufficient 
degree of accuracy for such practical work as fumigation. In order 
to calculate the cubic contents of an object it must be considered as 
shaped like some regular geometrical figure or figures. The figure 
which most closely approximates in shape an orange or lemon tree 
before it has been pruned is a cylinder surmounted by a hemisphere, 
and in computing the volume of citrus trees they are considered to 
be of this shape. 
If the height and width of a tree covered with a tent is known, it 
is a comparatively simple matter to calculate its contents. In the 
past work in California the dosage has been based upon these two 
measurements. After a tree is covered with a tent it is a matter of 
some difficulty to determine its height and width. By using as fac- 
tors the distance around the bottom of the tent and the longest dis- 
tance over the top of the tent we arrive at a more practicable method 
by which to compute the cubic contents, of a given tree. Using these 
