428 Proceedings of the Royal Irish Academy. 



They were made by means of the whii'ling machine invented by 

 Eobius 130 years ago, and subsequently used by other English phy- 

 sicists for experiments on the air's resistance. This apparatus was 

 established in the Hall of the Central Physical Obserratoiy at St. 

 Petersburgh on a grand scale. Its horizontal anns were 11 "26 feet 

 long, braced to prevent flexure, and 20-3 feet above the floor. The 

 Anemometers were attached to one of the anns at 10*92 feet from the 

 centre of rotation, and, except in one instance, with their planes of 

 rotation parallel to that of the arms. The number of turns of the 

 latter was recorded by an electric register, and the seconds of each 

 experiment noted with a chronometer. These gave the velocity with. 

 which the axis of the Anemometer passed through the air. It was 

 moved, in a way not corresponding to the general excellence of its 

 details, by two men impelling opposite bars projecting from the axle ; 

 and therefore the velocities communicated to it were not very uniform. 

 The greatest speed obtained was 40 kil. = 25 miles per hour. 



The velocity with which the centre of its cups revolved was given 

 by the number of its revolutions and its radius. ]\I. Dohrandt was 

 well aware of the cii'cumstances which make a difference between 

 the motion of an Anemometer carried through quiescent air, and of one 

 acted on in a fixed station by a current of wind ; and he showed great 

 sagacity and experimental skill in trying to eliminate the influence of 

 the two most important of them. 



Pirst, it is obvious that a cup which is within the track of its axis 

 meets the air with less velocity than one outside, and therefore receives 

 a less impulse than when outside ; fi-om which it follows that when the 

 machine revolves in the same direction with the Anemometer, the 

 latter revolves more slowly than in the reverse case. He, therefore, in 

 every case took the mean result of the two directions. 



Secondly, the rotation of the apparatus di'ags with it a quantity of 

 air, producing a circular current which is sensible even at the floor 

 20 feet below ; therefore the Anemometer meets the air with a less 

 velocity than if that were quiescent. He measui'ed this draught by 

 a " "Woltman's Ply," a light windmill, established with its axis nearly 

 in the plane of the Anemometer's rotation, and parallel to the tangent 

 of its track, and from 12 inches to 20 inches fi'om that track. He 

 estimated its velocity to be about 0"05 of that of the machine, and, 

 allowing for it, concluded that V, the velocity with which an Anemo- 

 meter is moved through still air, is connected with v the velocity of 

 the centre of the cups by the equation V=a-^ iv (inkilom.) 



He determined these constants for five Anemometers by the "whirl- 

 ing machine ; and four others by comparison with them. I give 

 their values for the first set, adding for each the length of its arms 

 and diameter of its cups in inches. 



