FUMIGATTOX 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 Whit tier, 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 (PL 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 



1 See Bui. 79, Bur. Ent., U. S. Dopt. Agr., p. 47, 1909. 



