172 Prof. G. G. Stokes. Discussion of the Results [May 12, 



were made in a building, which has the advantage of sheltering the 

 anemometer from wind, which is always more or less fitful, but the 

 disadvantage of creating an eddying vorticose movement in the whole 

 mass of air operated on ; whereas in the ordinary employment of the 

 anemometer the eddies it forms are carried away by the wind, and the 

 same is the case to a very great extent when an anemometer is 

 whirled in the open air in a gentle breeze. Thus, though Dr. 

 Robinson employed among others an anemometer of the Kew pattern, 

 his experiments and those of Mr. Jeffery are not duplicates of each 

 other, even independently of the fact that the axis of the anemometer 

 was vertical in Mr. Jeffery's and horizontal in Dr. Robinson's experi- 

 ments ; so that the greater completeness of the latter does not cause 

 them to supersede the former. 



In Mr. Jeffery's experiments the anemometers operated on were 

 mounted a little beyond and above the outer edge of one of the steam 

 merry-go-rounds in the grounds of the Crystal Palace, so as to be as 

 far as practicable out of the way of any vortex which it might create. 

 The distance of the axis of the anemometer from the axis of the 

 " merry " being known, and the number of revolutions (n) of the latter 

 during an experiment counted, the total space traversed by the anemo- 

 meter was known. The number (N) of apparent revolutions of the 

 anemometer, that is, the number of revolutions relatively to the merry, 

 was recorded on a dial attached to the anemometer, which was read at 

 the beginning and end of each experiment. As the machine would only 

 go round one way, the cups had to be taken off and replaced in a reverse 

 position, in order to reverse the direction of revolution of the anemo- 

 meter. The true number of revolutions of the anemometer was, of 

 course, N + or 1ST — n, according as the rotations of the anemometer 

 and the machine were in the same or opposite directions. 



The horizontal motion of the air over the whirling machine during 

 any experiment was determined from observations of a dial anemo- 

 meter with 3-inch cups on 8-inch arms, which was fixed on a wooden 

 stand in the same horizontal plane as that in which the cups of the 

 experimental instrument revolved, at a distance estimated at about 

 30 feet from the outside of the whirling frame. The motion of the 

 centres of the cups was deduced from the readings of the dial of the 

 fixed anemometer at the beginning and end of each experiment, the 

 motion of the air being assumed as usual to be three times that of the 

 cups. 



The experiments were naturally made on fairly calm days, still the 

 effect of the wind, though small, is not insensible. In default of 

 further information, we must take its velocity as equal to the mean 

 velocity during the experiment. 



Let Y be the velocity of the anemometer (i.e., of its axis), W that 

 of the wind, the angle between the direction of motion of the 



