10 EXPERIMENTS IN AERODYNAMICS. 



on the elevation at A', where it is shown as geared to this vertical axis by a pair 

 of bevel-wheels, that of the shaft having 15 teeth and that of the turn-table axis 

 having 7o teeth, or 1 to 5. The cone-pulleys used from the beginning of the 

 experiments up to September, 1890, have four steps with diameters of 211, 18i, 

 lit, and 8 inches. The speeds given by these pulleys in terms of whirling-table 

 revolutions for 1,000 revolutions of the gas-engine are approximately — 



Lowest speed 25 



Second " .50 



Third " 100 



Highest " 200 



The gas-engine speed varied from 180 to 190 revolutions per minute. 



In September, 1890, the above-described pulleys were replaced by a larger 

 set of three steps, having diameters of 36, 25* and 18 inches, respectively, which 

 give speeds in the ratio of 4, 2, and 1, and the gear, which liad broken, was 

 replaced by a new one of 1 to 4. 



This system gives for 120 revolutions of the steam-engine per minute, 

 driving — 



18 in. pulley, 48 revolutions of turn-table per minute = 100 -f miles per hour at end of arm. 



25J " " 24 " " •■ =.50 + ■' 



36 " " 12 " " " = 25 + " " " " 



By regulating the speed of the engine any intermediate velocities can be 

 obtained, and thus the equipment should be susceptible of furnishing speeds 

 fi'om 10 to 100 miles per hour (4.5 to 45 meters per second) ; but owing to the 

 slipping of belts the number of turn-table revolutions was less than this for 

 the higher velocities, so that the highest attained in the experiments did not 

 reach this upper limit, but was a little over 100 feet (30 meters) per second, or 

 about seventy miles per hour. The precise velocity actually attained by the 

 turn-table is determined, quite independently of the speed of the engine, by an 

 electrical registration on the standard chronograph in the observatory. The 

 electrical current passes into four fixed contact-pieces (shown at 0-P, plate II, 

 and on large scale in plate III) fastened to a tixed block placed around the 

 axis of the whirling table, these fixed pieces being placed symmetrically around 

 the axis, while another platinum contact-piece is fastened to a horizontal arm 

 screwed into the axis of the turn-table and revolving with it, thus " making 

 circuit " every quarter revolution of the table. The current passes out of the axis 

 through a brush contact, shown in plate III, and thence to the chronograph in the 

 observatory. C designates the fixed contact pieces, and P the platinum piece 

 revolving with the axis. S and L are adjusting screws. Turning again to plate 

 II, an additional brush contact, shown at B, and again at B', serves to transmit 



