230 



SCIENCE 



[N. S. Vol. LI. No. 1314 



have made some progress since Galileo's time. 

 We know that bodies are retarded by the air 

 but we have assumed, on some experimental 

 evidence, in the case of projectiles at any 

 rate except for a constant of proportionality, 

 that they are retarded in the same way. It 

 is evident that in the matter of the laws of 

 air resistance we are not far from the condi- 

 tion that the scientists of Galileo's time were 

 in regard to gravitation. 



It is evident from the results of these ex- 

 periments at Aberdeen that a very slight 

 change in the form of the projectile may , 

 make a considerable change in the range ob- 

 tained. And it is equally clear that those 

 experiments merely touched the matter. The 

 entire subject is still open. 



A number of years ago the Ordnance 

 Department made inquiries concerning the 

 possibility of using air streams of high 

 velocity in tests on projectiles. During the 

 war the project was submitted to the National 

 Research Council. It was found that air 

 streams one foot in diameter, with speed of 

 1,500 feet per second, requiring for their pro- 

 duction 5,000 kw., could be furnished by the 

 General Electric Company at their plant at 

 Lynn, Massachusetts. There, with the most 

 loyal support of the Bureau of Standards, and 

 with the effective collaboration of Dr. L. J. 

 Briggs of the bureau the Ordnance Depart- 

 mient has conducted experiments^ which have 

 for their object the determination of the 

 forces of such air streams on projectiles of 

 various forms. Velocities of the air have, so 

 far, varied from 600 up to 1,200 feet per 

 second and temperatures from 0° to 130° C. 

 In these air streams, which are vertical, pro- 

 jectiles of various shapes can be held nose 

 down, and the forces on them and pressures 

 at various points on their surfaces, can be 

 measured. A number of important results 

 have been secured. First, for head-on resist- 

 ance there is no one curve similar to the 

 French B curve which gives the law of air 



3 Wiitkout a knowledge on his part of other in- 

 quiries, negortuations for these experiments were 

 carried on and pushed to a conclusion by Major 

 Moulton. 



resistance for all projectiles. For example, in 

 that law it will be seen by inspection (Fig. 1) 

 that F/v^ is multiplied by the factor 3 when 

 the velocity changes from 200 to 380 meters 

 per second. In our curves the corresponding 

 factor varies from 1.3 to 4 for the various 

 forms of projectiles. In other words the 

 force exerted on one projectile may be less at 

 one velocity and more at another than the 

 force for the corresponding velocity in the 

 case of another projectile. It follows that 

 there is no " best form " of projectile unless 

 we specify the approximate velocity with 

 which we are dealing. 



Second, the results obtained indicate the 

 resistance introduced by the rotating band 

 and show where this band should be placed 

 to produce the least increase of resistance. 



Third, it appears that the rapid rise of the 

 B curve in the neighborhood of V = 340 

 meters per second is not entirely determined 

 by the velocity of the compressional wave, i. e., 

 by the velocity of sound in the air. In some 

 cases the force of air streams at 130° C. are 

 identical with those at 30° C. (It is understood 

 that the density of the air is standardized, i. e., 

 that the forces plotted are those which an air 

 stream of equal speed and of density 0.001206 

 gms./em.' would have exerted.) In other 

 cases, however, the results indicate that the 

 velocity of the compressional wave is one of 

 the factors determining the resistance. The 

 temperature relation seems to be a compli- 

 cated one and our results are not at all com- 

 plete on this point. 



Fourth, thoTigh we have not made quantita- 

 tive measurements of the variation of force 

 with the angle of attack of air and projectile, 

 we have had some experimental evidence of 

 the large forces which are called into play 

 when this angle changes from " nose on " to 

 oblique. In one case, the force of the air on 

 a fifty pound 4-inch projectile was of the 

 order of 44 pounds, so that there was still 

 about six pounds of down force. When the 

 projectile was being removed from the air 

 stream it was accidently tipped slightly. The 

 air stream forced it farther from the vertical, 

 bent off the steel rod holding it to the balance 



