26 BULLETIN 875, U. S..DEPARTMENT OF AGRICULTURE. 
The driving mechanism should be so arranged that the fan will - 
continue in operation while the machine is turning on one wheel in 
either direction at row ends. 
The running and driving mechanism should be shielded to pre- 
vent plant injury or clogging with vegetation. 
The method of hitching should allow flexibility which will pre- 
vent the load from being thrown on one animal. 
All parts should be readily accessible for repair or replacement. 
POWER DUSTER. 
.The engine of a power duster should be as simple and efficient a 
type as possible and should provide a slight surplus of power over 
that actually required to operate the niacin. 
The truck should be provided with a fifth wheel ceeeipemeaen and 
the front wheels should cut under the body to permit short turns. 
The distributor construction should be strong enough to carry the 
wide spread of nozzles. 
COST OF POISONING. 
Anyone who has read the preceding pages carefully can easily 
appreciate the futility of attempting to give generalized figures on 
the cost of poisoning which would be of any value to the individual. 
The cost to each man is going to be an individual problem and 
will differ radically in the different fields and in different seasons. 
Although careful cost figures have been kept in connection with the 
experiments which have been conducted in the past on both large 
and small scale work, it is useless to give more than a few generalized 
statements concerning these. Where the operation has been con- 
ducted on an individual field basis requiring in the neighborhood of 
four applications, it has been found that the cost of poison, labor of 
application, and reasonable depreciation on the investment for ma- 
chinery has been in the neighborhood of $7 to $10 per acre for the 
season. On a large plantation basis the investment in poison and 
machinery has naturally been lower but the additional cost of super- 
vision, etc., has generally been sufficient to offset this difference. At 
present prices, it is hardly safe to figure on a cost of less than $2 an 
acre per application. 
GAINS TO BE EXPECTED FROM POISONING. 
The gain in amount of seed cotton to be expected from the use of 
poison is another point which is not subject to definite figures. Dur- 
ing the past five years several hundred plat tests have been con- 
ducted in such a manner that an accurate comparison in yields was 
de, 
