INSECTICIDES AND EQUIPMENT FOR CONTROLLING INSECTS 43 
Compressed-air sprayers.—Compressed air sprayers (fig. 2) are 
usually made of brass or galvanized sheet steel and have a capacity 
ranging from 1 to 5 gallons. The sprayer consists of an airtight tank 
into which is clamped a pump. In operation the tank is filled with 
spray to about three-fourths of its capacity, and the opening is closed 
by a tight-fitting cap. Air is then pumped in by hand until sufficient 
pressure is developed to force the liquid through the hose and nozzle. 
The hose is usually fitted with a spray shut-off and a 1- to 5-foot exten- 
- sion rod with nozzle. Since the pressure decreases as the spray is 
forced out, it is necessary to renew it by further pumping. The tank 
must be shaken frequently to keep the ingredients well mixed, al- 
though the movement of the operator causes a certain amount of agi- 
tation. 
Figure 2.—Compressed-air sprayer. 
This type of sprayer is suitable for the average home gardener. By 
the use of extension rods low trees can be sprayed. 
_ Knapsack sprayers.—A knapsack sprayer (fig. 3) consists essentially 
of a force pump with an air chamber fitted to a metal tank and so de- 
signed that it can be carried on the back of the operator. It can be 
operated by one hand while the other is used to manipulate the spray 
rod and nozzle with which the apparatus should be equipped. A knap- 
sack sprayer has a capacity of about 3 to 5 gallons and a spraying 
range of about 25 feet when equipped with a nozzle throwing a com- 
pact stream of spray. It has an advantage over the compressed-air 
sprayer in that a higher and more uniform pressure can be maintained 
when the pump is kept in constant operation. Agitation is provided 
in some outfits by a brass plate inside of the tank which moves up 
and down with the pump handle. 
The usefulness of knapsack sprayers corresponds closely to that of 
compressed-air sprayers. They can, however, be used on larger plant- 
ings and on somewhat larger trees. 
