6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



tunnel. The screw sucks the air of the closed room through the 

 mouth of the tunnel, which is somewhat flaring, thence through a 

 metal honeycomb, into the experimental part of the tunnel where the 

 models are placed for study. Thence the air flows into the expanded 

 half of the tunnel, passing first through the screw, then laterally 

 outward through innumerable holes in the tunnel wall, thence in uni- 

 form circulation through the unobstructed room till it curves again 

 easily into the mouth at the opposite end. The air stream so produced 

 is, where it emerges from the honeycomb, uniform in velocity at all 

 parts of a section, at least to a fraction of one per cent, if due care 

 be taken. The expanded and perforated part of the tunnel is said 

 to be the final outcome of long months of trial and study by the tech- 

 nical stafif, and has enabled them to produce the steadiest aerodynamic 

 current in the world ; thus removing one of the greatest difficulties 

 in the accurate determination of the flow and pressure of air about 

 wind models. The current velocity is reported to be uniform to 

 one-half per cent both in time and in space. 



The complete structure of the tunnel need not be delineated here, 

 as it may be had better from the general plans and detailed working 

 drawings which the director of the laboratory has kindly ofifered to 

 furnish the Smithsonian Institution. It may be explained, however, 

 that the " honeycomb," just within the flaring mouth of the tunnel, 

 consists of crossed metal sheets forming, post-office-box-like, a 

 tubular partition of many cells through which the air entering the 

 tunnel is straightened and deprived of eddies. It may also be ob- 

 served that a glass door is placed on the side of the tunnel, through 

 which one may take observations, or enter to adjust the models to 

 be tested. 



The cost of the seven- foot wind-tunnel is given as about $2,000, 

 and of its wind balance about $2,000. This, with an expenditure of 

 $12,500 for the building, makes a total of $16,500 for the plant. 



The velocity of the air flow in the unchecked current, near the 

 model held inside the tunnel, is computed from the observed pressure 

 difference between the inside and outside of the tunnel wall. The 

 accuracy of this method was experimentally proved by me in 1902 at 

 the request of the Navy Department, and, together with a mathe- 

 matical proof, was set forth in the Physical Rcviczu the following 

 year. It was there shown that the speed of air rushing steadily 

 through a horizontal cylindrical tube from the quiet atmosphere of 

 the room into a chamber at low pressure is, for ordinary trans- 

 portation speeds, given truly to a fraction of one per cent by the 



