203 Prof. R. S. Pall on the Resistance of Air 



centre of disk, then we have 



Pr=|>^) =2oyJ=2a>,n>; 



,\ P = 2(OjV = 2co sin \ . v. 

 Hence, for horizontal motion in a great circle, the tendency to 

 swerve is rigorously equal for all directions of motion, a result 

 which will be found-to agree with the comparison of II. and V. 



XXV. Account of Experiments upon the Resistance of Air to the 

 Motion of Vortex-rings. By Robert Stawell Ball, A.M., 

 Professor of Applied Mathematics and Mechanism, Royal Col- 

 lege of Science for Ireland, Dublin*. 



THE experiments, of which the following is an abstract, were 

 carried out with the aid of a grant from the Royal Irish 

 Academy. A paper containing the results has been laid before 

 the Academy. A brief account of one series of the experiments 

 and a Table embodying them will be given. 



Air-rings 9 inches in diameter were projected from a cubical 

 box, each edge of which is 2 feetf. The blows were delivered 

 by means of a pendulum called the striker, which, falling from 

 a constant height, ensured that the rings were projected with a 

 constant velocity. In the experiments described in the present 

 series, this velocity was a little over 10 feet per second. The pen- 

 dulum was arranged so as to open a circuit at the instant of its 

 release. 



After the ring had traversed a range which varied from 2 

 inches to 20 feet, it impinged upon a target. The blow upon 

 the target closed the circuit, which had been opened at the release 

 of the striker. A chronoscope measured the interval of time 

 between the release of the striker and the impact upon the target. 



The target was placed successively at distances of 2, 4, 6, 8, 

 10, 12, 14, 16, 18, 20 feet from the orifice of the box. Not less 

 than ten observations of the time were taken at each range. The 

 probable error of the mean time at each range is in every case 

 less than 1 per cent, of the whole amount. A special series of 

 experiments, which need not be described, determined the value 

 of the chronoscope readings in seconds. 



The observations are next represented in a curve, of which the 

 abscissa? are the ranges, and the ordinates the corresponding 

 mean chronoscope readings. By drawing tangents to this curve, 



* Communicated by the Author, being an abstract of a paper read before 

 the Royal Irish Academy. 



t This method was suggested by Professor Tait, See a paper by Sir 

 William Thomson, Phil. Mag. July 1867 ; also a paper by the author, Phil. 

 Mag. July 1868. 



