90 SPRAY MACHINERY — GENERAL PRINCIPLES 
applied that it presents a perfectly true surface for the piston, and if the 
enamel is so baked on that it will not chip or wear rough from the grit 
that invariably gets into spray material, such cylinders should prove 
satisfactory. 
A pump must have an efficient mechanical agitator, which will 
automatically keep the liquid thoroughly stirred. A jet of liquid 
forced out in the vicinity of the intake is insufficient. Agitators of 
the paddle type, connected in some fashion to the pump handle or piston 
rod, are in general use, and are fairly effective. Another style of agi- 
tator works on the principle of a propeller, and gives excellent service. 
The size of air chamber is important. If it is large, the operator of 
the pump will find that a steady pressure can much more easily be 
maintained. Power pumps are apt to have air chambers of proper size, 
because if lacking, they will quickly tell on the working of the engine, 
specially if the latter is of the single cylinder type. Some of the smaller 
hand pumps would be more efficient and would work more easily if pro- 
vided with a more generous air cushion. It should be remembered 
that the spray liquid is only to a minute degree compressible, whereas 
air is resilient and absorbs the variations in pressure between strokes. 
Valves are necessarily a part of every pump, and are of various types. 
Those in commonest use are the poppet valve, the swing check, and 
the ball valve. Of these the ball valve is apt to give the best satis- 
faction, if it is made of proper materials. The action of the liquid tends 
constantly to turn the ball this way and that as it rises and falls, resulting 
in equal wear all round, both as regards the ball and the seat. Poppet 
valves with a single rod guide beneath to hold them in line are very likely 
to wear unevenly and later to leak, resulting in weakened efficiency. 
Any valve with a straight or square, instead of a beveled seat is apt to 
give trouble. All poppet valves present the following difficulty: 
they tend to seat in identical positions. and if grit or other foreign 
matter gets in, there is leakage at one point, which does not promptly 
right itself and soon results in uneven wear. It is essential to the 
satisfactory working of a pump that its valves fit accurately and do not 
leak. 
Ease of getting at the working parts of the pump is worthy of con- 
