212 SHADE-TREES IN TOWNS AND CITIES 



come into general use during the last few years. This type 

 of machine is very easy to operate. The power is supplied 

 by liquid carbon dioxid contained in steel tubes connected 

 with the steel insecticide tank by means of a piece of hose. 

 When the valve of the carbon dioxid container is opened, 

 the liquid in passing through the gas-tube turns into a gas 

 which creates the pressure in the insecticide tank. The 

 amount of the pressure is indicated by a pressure gage. 

 Sufficient force can be supplied by the carbonic-acid gas to 

 spray the tallest trees. 



Gas-sprayers are good, but require a supply of gas con- 

 stantly available, and the actual cost of operation is very 

 expensive. The liquid carbon dioxid costs about a dollar 

 for every one hundred and fifty gallons of insecticide used. 

 On the average, a man sprays about one hundred and fifty 

 gallons of insecticide mixture per day, so that with three 

 nozzles working, the operating cost of gas-sprayers is about 

 three dollars per day. The operating expense of gasoline- 

 sprayers is trifling— only a few cents a day. The simplicity 

 of gas-sprayers, however, makes them frequently desirable 

 even in spite of the additional operating cost. So long as 

 there is a supply of carbon dioxid on hand there is no loss 

 of time during the working day. Gasoline-sprayers some- 

 times get out of order, and the loss of time for repairs is con- 

 siderable. If a good mechanic is available to run a gasoline- 

 sprayer there is little loss of time; but a gas-sprayer gives 

 better results in less skilled hands. 



The agitator^on the gas-sprayers is of the mechanical 

 type, operated by a crank. Care must be taken to see that 

 the man turns the crank every minute of the time that the 

 machine is in operation if a mixture is used that will settle. 



Spraying Hose.— The very best hose made is none too 



