182 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



velopments in rocket propulsion have made it possible to carry instru- 

 ments up to heights of as much as 200 km. to make direct measurements 

 of local conditions in the high atmosphere.^ Because of the peculiar 

 difficulties associated with use of rocket-transported instruments, the 

 methods of observation often have to be very different from those that 

 would normally be used in a laboratory measurement. Considerable 

 ingenuity has been exercised in the design of rocket experiments and 

 very important new results are now emerging. It is interesting and en- 

 couraging to note that, at least below 100 km., direct observations made 

 by the use of rockets has largely confirmed the deductions from the 

 indirect methods. The technique developed by Paneth and his co- 

 workers for the microanalysis of gases (Endeavour, vol. 12, p. 5, 1953) 

 has proved very useful in investigating samples of air recovered from 

 rockets sent to great heights. 



PRESSURE, TEMPERATURE, AND COMPOSITION 



If the composition of the atmosphere is known at any altitude, meas- 

 urements of any two of the three quantities pressure (p), density (p), 

 and temperature (T) enable the third to be deduced. Up to about 100 

 km. it is probable that there is no appreciable change of composition 

 from that at ground level, and this is consistent with the observations 

 of p, p, and T by various methods. 



The pressure and density have been measured directly up to altitudes 

 as high as 200 km. The pressure is determined by suitable pressure 

 gages (bellows gages for low altitudes and ionization gages for the 

 high ones) inserted on the sides of a rocket near the tail fins. Wind- 

 tunnel tests have confirmed that such gages measure the ambient pres- 

 sures. The density is determined from the stagnation pressure, meas- 

 ured by a suitable pressure gage at the nose of the rocket. The mean 

 values of results obtained to date are given in figure 2. 



The temperature variation has been determined by several methods. 

 If the velocity, v, of sound can be obtained, the temperature can be 

 calculated from the relation 



V^ = yRT/M 



where R is the universal gas constant, M the average molecular weight 

 of the air, and y the ratio of the specific heats of the gas. The first way 

 in which this possibility was utilized concerned the so-called anomalous 

 propagation of sound. Intense sounds, such as those of gunfire, are 

 often heard at great distances from the source, beyond an intervening 

 zone of silence. This is because sound waves traveling at large inclina- 

 tions to the horizontal are reflected by a high-temperature region, in 



' A detailed account of investigations of this kind will be found in the Proceed- 

 ings of the Oxford Conference on Rocket Exploration of the Upper Air, Pergamon 

 Press, London, 1954. 



