particle sizes. This can be done by forming ratios of the various forces and see what 
functions of drop diameter results. However, these particle forces always accelerate 
the particles to some velocity where aerodynamic drag equals the force. Since aerody- 
namic drag is always present, another useful comparison is to compare the equilibrium 
velocity due to the force in question with the equilibrium velocity of settling in still 
air due to gravity. 
The ratio of aerodynamic force to gravitational force is: 
Bayh 18uv 
5S TRL) 
Bg) (Os aati DE 
The ratio of the maximum force due to electrical charges to the gravitational force 
is given by: 
F 
eo = (eG 2c Oe aeeve 
g PpD 
The ratio of a force due to thermal gradient to force of gravity is given by: 
Ey 18 MR xq VT 
ine pMp (Bsc. db =e 
The ratio of the maximum terminal velocity due to electrical forces to the terminal 
velocity due to gravity is given by: 
Ve (2.65 x 10-9) VV 
Veg D Po 
The ratio of maximum velocity due to a thermogradient to maximum velocity due to 
gravitational attraction is given by: 
2 
Meas Zi eR AV 
Vie 08 D2pM(3x, + xp) 
With all these forces available and apparently explicit formulas as to how to obtain 
them, one might wonder what is preventing us from putting sprays and dusts exactly 
where we want them right now. 
Our statement of the problem at the beginning of the paper says that we must apply 
these chemicals with practical equipment. Practicality involves at least two things-- 
capacity and cost. For instance, if we devise equipment which would yield highly im- 
proved deposition efficiencies travelling at only a half mile per hour, it would not be 
considered practical equipment. Likewise if spraying and dusting equipment with greatly 
improved deposition efficiency were to have greatly increased initial cost or greatly 
increased cost of operation, it could not be considered practical. Even with the inexpen- 
sive spraying equipment available now, the cost of application, including both the cost 
of operator time and amortization of equipment, is often equal to, or greater than, the 
cost of chemicals applied, when such commonchemicalsas 2,4-D and 2,4,5-T, are used. 
Almost everything described so far is concerned with single particles of a particular 
size. In practice we likely will have a cloud of thousands of particles of a number of 
sizes. We know for instance that aerodynamic forces interact on particles nearby so 
that the path of a single particle travelling through the air subjected only to aerody- 
mamic forces is not the same as several particles which are close to one another. 
80 
