FUMIGATION OF CITRUS TREES. 33 
and that the distance around the tent be secured by means of a tape. 
To meet this requirement, a scheme has been devised by Messrs. 
Griffin and Gray, of Whittier, CaL, which renders the securing of the 
distance around the tent not only absolutely accurate but also more 
rapid and easy than by pacing. The apparatus consists (1) of a 
straight iron rod 3 or 4 feet long and about one-half inch in diameter, 
having the lower end sharpened while the upper end is made in the 
form of a loop, and (2) a strong tapeline having a snap at one end by 
which it is fastened to the loop of the iron rod. 
To secure the distance around a tent the iron rod is stuck into the 
ground at one end of the marked line on the tent which runs over the 
top of the tree (PI. V, fig. 6). The operator then moves around 
the tree, allowing the tape to slip through his hand as he moves. 
When he has obtained the distance around he drops the tape, takes 
the iron rod, with the tape attached, to the next tree, and continues 
as before. In this manner the operator is required to move only 
once around each tree. This method is entirely practical, as proved 
by experience, and in having their work done the growers should 
demand its use. It reduces variation resulting from the work of 
careless operators to a minimum. From these two measurements 
(the distance around and the distance over) it is possible to approxi- 
mate the cubic contents of the tree and thereby calculate the dosage. 
This might be done in the field and the trees then dosed in pro- 
portion to the contents. However, the time required for the cal- 
culation of the dosage, even after determining the cubic contents 
of the tree, would not only prevent rapid field work and allow an 
opportunity for error, but would cause a lack of uniformity in dosage 
from the consideration of the cubic contents alone, as will be explained 
later. This difficulty has been obviated by preparing a dosage 
schedule from which the required dosage may be calculated without 
any figuring as soon as the measurements of the tree are known. 
LEAKAGE OF GAS. 1 
One of the most important questions relating to the proper dosage 
in fumigation is that of leakage of gas through the tent. In fact, 
with the present character of tenting, where the gas has usually all 
escaped by the end of an hour, the dosage depends directly on the 
amount of this leakage. In figures which approximate a citrus tree 
in shape the volume decreases at a more rapid rate than does the 
surface. Computation shows that a tree 20 feet around by 12 feet 
over has 0.86 of a square foot of tent surface for each cubic foot of 
gas within to escape through, whereas a tree 79 by 54 feet has only 
0.22 of a square foot of tent surface for each cubic foot of gas to 
escape through. This would mean that there is about four times as 
Bui. 79, Bur. Ent., U. S. Dept. Agr., p. 47, 
