142 



ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1908. 



known at the date of his writing, 1873; but he could not take ac- 

 count of the sliding, or skin-frictibn, because in his day neither 

 the magnitude of such friction for air, nor the law of its variation 

 with velocity, had been determined. 



Skin-friction in air. 



Even as late as Langley's experiments, skin-friction in air was 

 regarded as a negligible quantity, but due to the work of Doctor 

 Zahm, who was the first to make any really extensive and reliable 

 experiments on skin-friction in air, we now can estimate the magni- 

 tude of this quantity. As a result of his research he has given in his 

 paper on " Atmospheric friction " the following equation : 



/= 0.00000778 Z~°°^ v-^^. . . (v = feet per second), 

 /= 0.0000158 r°-°''y'-'^ 



(v = miles per hour). 



in which / is the average skin-friction per square foot, and I the 

 length of surface. 



From this equation the accompanying table of resistances w^as 

 computed, and is inserted here for the convenience of engineers: 



Table 2. — Friction per sqiiai-e foot for variotis speeds and lengths of surface. 



The numbers within the rules represent data coming within the 

 range of observation. These observations show that " the frictional 

 resistance is at least as great for air as water, in proportion to their 

 densities. In other words, it amounts to a decided obstacle in high- 

 speed transportation. In aeronautics it is one of the chief elements 



