250 
ZAHM. 
quires less than half an hour, and the plane can easily be 
made true to less than 3 V of an inch. The warpage of the 
16-foot plane sometimes amounts to one-eighth of an inch 
in twenty-four hours, and may be more than a quarter of 
an inch in several days; but in practice the plane is kept 
straight by timely adjustment. 
During each experiment one assistant controlled the fan 
speed by means of a rheostat, and noted the revolutions per 
minute with a Schaeffer and Budenberg tachometer; another 
assistant read the deflection of the plane, while a third ob¬ 
served the differential pressure in the wind-shields by 
means of a manometer, and the wind velocity as given by a 
pressure-tube anemometer or a Robinson cup anemometer. 
The duration of an experiment was usually about an hour 
and comprised ten different wind velocities. 
Fig. 2. — Cross-section of 16 -foot Plane , Showing Paper Glued to Sliding Pieces. 
The following page from the laboratory note-book for 
January 30, 1903, gives the results obtained after some skill 
had been acquired in using the various instruments. Similar 
observations had been taken in July, 1902, and this much 
of the present paper was communicated to the American 
Association for the Advancement of Science in December, 
1902. 
A few essential data may be prefaced: surface of plane 
between w 7 ind shields, 138.08 square feet; cross-section of 
plane, 202.1 square inches ; weight, 58 pounds; 1-inch swing 
of plane, =0.296 pound deflecting force; 1 milligram per 
square centimeter differential pressure in the wind shields 
equals 0.00287 pound end-thrust on the plane; mean tem¬ 
perature of experiment, 4°.5 C.; barometric pressure, 29".74; 
time, 3.30 to 4.30; weather, dry; mouth of tunnel not 
screened. 
