CHARACTER AND METHOD OF EXPERIMENTS. 11 



a current to wires running out to the end of tlie wliirling arm, so that seconds 

 from the mean time clock and other phenomena can be registered on the recording 

 cylinder of the dynamometer chronograph at the end of the arm; and also 

 phenomena taking place at the end of the arm can be registered on the chrono- 

 graph in the observatory. By these means the experiments are put under 

 electric control and perfect knowledge is obtained of the velocity of the turn- 

 table at the moment when any phenomenon occurs. This brush contact was 

 made sufficiently large and heavy to transmit a current from a dynamo to an 

 electric motor placed on the whirling arm, and, having this electric equipment 

 extending to the outer end of the whirling arm, different pieces of apparatus 

 were devised for registering pressure and other phenomena there. 



The whirling table was thus established and the experiments conducted in 

 the open air, not through choice, but because the erection of a large building 

 specially designed for them was too expensive to be practicable. It was hoped 

 to take advantage of calm days for the performance of experiments, as in a calm, 

 a whirling table in the open air is under the best possible conditions, for in a 

 confined building the rotating arm itself sets all the air of the room into slow 

 movement, besides creating eddies which do not promptly dissipate. Practically, 

 however, these calm days almost never came, and the presence of wind currents 

 continued from the beginning to the end of the experiments, to be a source of 

 delay beyond all anticipation, as well as of frequent failure. 



In the latter part of April, 1889, an octagon fence 20 feet high (shown on 

 plate I) was erected around the whirling table with the object of cutting off, to 

 some extent, the access of the wind. This, however, proved to be ineffectual, and 

 the difficulty experienced from the wind continued nearly unabated. 



If any one should propose to repeat or extend tliese experiments, I would 

 advise him, first of all, and at all costs, to establish his whirling table in a large, 

 completely inclosed building. 



