214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 



next device, shown in plate 2, figure 2. Note that two surfaces are 

 attached to a trailing arm in wind-vane fashion and each can be 

 adjusted until their opposing lifts balance. Later correspondence 

 brings out the fact that this was their first attempt at using a wind 

 tunnel and that it was actually made by knocking the ends out of a 

 box used in those days for shipping laundry starch. An air blast was 

 supplied from the opposite end by a screw fan turning 4,000 revolu- 

 tions per minute. No doubt this was driven from their machine-shop 

 lineshaf ting, which, incidentally, was driven in turn by a 2-horsepower 

 gasoline engine of their own design. 



With this device they experimented with various aspect ratios and 

 curvatures. One particular observation that was recorded was the 

 balancing of a 1- by 3-inch curved surface at 4%° against a flat plane 

 of the same area at 91/2°, whereas their reference tables indicated 

 they should have balanced at angles of 4%° and 24° respectively. 

 The correspondence files include a letter to Chanute, wherein Wilbur 

 pointed out a number of these discrepancies. Chanute very promptly 

 answered back that they were comparing results taken from moving 

 wind measurements against measurements that had been made in still 

 air. It was rather amusing to note Wilbur's reply that this should 

 make no difference, although he did temper his brusqueness by explain- 

 ing how easy it would have been for the particular investigator to 

 have made mistakes by the method he was using. 



Compared with the first device, this method was far more accurate 

 and served to make many more comparisons in a short time ; however, 

 it still did not provide the means of making direct measurements and 

 was soon abandoned. 



The third and final type of measuring instrument was evidently 

 built and put into operation sometime between October 16 and No- 

 vember 14, 1901. During that time they built a tunnel like that shown 

 in plate 3, figure 1. This is as near to an exact replica as it is possible 

 to build from the available information. The lift instrument (see pi. 

 3, fig. 2) , which was placed at the downwind end, is an exact copy of 

 the original, which is now on display at the Franklin Institute in 

 Philadelphia. 



Perhaps the most unique feature of this instrument is the way in 

 which the lift of a model surface is made to balance the flat-plate 

 resistance of four small fingers on the lower bar. The shackle arms 

 which support the upper crossbeam are snug on the vertical pins and 

 are adjusted so that they trail straight with the wind stream. Since 

 the resistance or lower beam must ride at some angle to the side in 

 order to balance the lift of the test surface, it is obvious that the sine 

 of the angle observed on the scale is the true lift coefficient. Angle 

 of attack was recorded by separate protractor. A two-bladed 24- 



