The second principle that emerged was that both processes became less effective with higher densities; in fact, we were forced to go to impaction on a screen for 175 kilofeet to attempt sampling of the debris of Orange shot. When we became interested in the sampling program above 100 kilofeet, these principles came to mind and we wondered if they might be exploited in the regime above that altitude, to discuss are an effort to explore these possibilities, The calculations that Iam going It should be emphasized that these calculations are preliminary and are intended to be a feasibility check and a guide for experimentinitiation. Furthermore, there is a crucial assumption in my calculations which must be borne out by experiment before one could confidently attempt to use it. You will remember that Dr, Martell and Dr. Davis have spoken of the sedimentary behavior of heavy molecules. I am assuming in my diffusive collection calculations that these molecules are a super-saturated vapor which will condense on any available surface. to check out this postulate. Mr. Rigali will mention how we hope For larger particles, we will probably impregnate our filter media with an ad- hesive on which the particles will be trapped. If we are going to examine diffusion and impaction collection, we must first estimate the amount of flow through the filter media. I have chosen to represent this filter as a system of parallel capillary tubes, calculations reveal that the filter must be extremely flimsy. Rough We have found that to obtain reasonably high flow rates, the length of the capillary should be only about ten times its radius, I think it will be more apparent how flimsy this means our filter media should be when we discover how small the pores must be for effective collection, The flow equations for capillaries and assemblies of capillaries are given in Figure 4. The top half of this illustration covers the flow equations for the free molecular region where the mean free path of an air molecule is large compared to the pore diameter. lary. The first equation describes the mass flow through a single capil- The second is our working equation which is derived by relating the dynamic pressure to the pressure drop across the filter and considering the flow through the assemblage of parallel capillaries. sumed to be incompressible. The flow is as- In this working equation, V is the average velocity normal to the "accordion" fil- ter entrance, a is the capillary radius, A is the fraction of the filter surface which is open for flow, and F is the fold factor, i.e., the cosecant of the filter accordion half-angle. gram of air, The term R is of the gas constant for one T is the absolute temperature, k is the ratio of the capillary length to its radius, while V is the sampler velocity. It should be noticed that the free molecular flow region has a nice feature. The flow velocity to the system for a given sampler velocity is independent of altitude if we ignore the temperature dependence which is practically not very important. This means that if we can always have the mean free path of the air molecules large compared to the capillary diameter, the velocity programing problem for the impellers is considerably simplified. In general we find that v, the normal flow into the filter area, is considerably less than the sampler velocity, This means that to secure isokinetic sampling we must provide for flow expansion from the sampler opening to filter region, In the lower portion of Figure 4 we have the equations which describe the Poiseuille flow through a capillary and an emsemble of capillaries. This expression is appropriate when the mean free path of an air molecule is sm2ll compared to the diameter of the capillary. Here again, the first equation describes the mass flow through a single capillary, while the second equation is our working equation derived in the same manner as we did for the free molecular flow region, same, 140 except we have two new terms: The meaning of the symbols ts the p, which stands for the average pressure, and » 2 which designates