( 359 ) 



XVII Results of Ohsei'vations made with WheweWs Anemometer'. ^^ Mr John 

 Rankine. Communicated hy Professor Fobbes. 



. Read 24th December 1838. 



The following observations of the direction and force of the wind, were made 

 with an anemometer lately invented by Professor Whewell of Cambridge. 



In this instrument the only part that is fixed is a japanned cylinder, on 

 which the points of the compass are marked by black lines dividing it in its whole 

 length into compartments, corresponding to the spaces intervening between any 

 two double points. On this the other parts are supported by means of a strong 

 rod which runs down the centre of the cylinder, and, terminating in a sharp 

 point, turns easily round as the wind changes. A single broad vane, having the 

 rod running down the centre of the cylinder for its axis, presents to the wind a 

 fly resembling the sails of a wind-mill, and causes the moveable part of the in- 

 strument to revolve round the fixed cylinder as the wind changes ; so that the 

 aerial current, come from what quarter it may, blows against the circular disk 

 of the fly, and turns it with a velocity proportional to the force of the wind at 

 the time. The motion thus produced is diminished by two endless screws work- 

 ing in the circumference of toothed wheels. The axis of the second wheel is 

 continued downwards nearly to the foot of the cylinder, where it is supported 

 and turns in a collar connected by a graduated rod with the upper part of the 

 instrument. A pencil, attached to a nut, descends on the axis of this wheel, and 

 presses against the surface of the cylinder, tracing in its progress, and as the 

 vane wavers, a thick irregular line like the shadings on the coast of a map : 

 the middle of this line is easily ascertained, and, from the compartment of the 

 cylinder on which the marks are made, shews to the eye at one view the average 

 direction of the wind, or, in other words, the point of the compass on either side 

 of which the wind continually oscillates. The length of the line is measured by 

 means of two indexes, which slide along the graduated rod connecting the upper 

 part of the instrument with the collar near the bottom of the cylinder. The de- 

 scent of the pencil, thus ascertained, is proportional to the velocity of the wind, 

 and the time dm-ing which it blows in one direction jointly. This gives what Mr 

 Whewell calls the Integral Effect of the wind, or the total amount of the aerial 

 current that passes over the instrument in any direction, dming the interval that 

 elapses between the recordings of its indications. The space through which the 



VOL. XIV. PART II. Z Z 



