308 Mrwhewell's results of observations 



for a long time with a known velocity ; and thus the actual value of 

 the indications of the instrument might be determined. And a small 

 Comparative Anemometer, more easily transferable from place to place 

 than the working instrument, might be employed to obtain the value 

 of the scale of the instrument in this manner. This process might be 

 performed at any time, and might therefore serve to compare the 

 Anemometer with itself at different times. The relation between the 

 velocity of rotation produced, in a wheel with oblique blades, and the 

 velocity of a fluid which flows past it, is so steady, that the rotation 

 of such a machine has already been used in measuring the velocity of 

 the motion, in Masson's Patent Log, and Saxton's Current-meter. 



The same process which would compare an instrument with itself, 

 would also compare it with another instrument of the same kind. But, 

 as we have not yet any such means of judging what is the comparative 

 going of different Anemometers, we may say a word or two of the 

 comparison of them by means of their results. The station at the 

 Society's house and the Observatory are so near each other, that there 

 can hardly be any great difference in the quantity of wind which blows 

 at the two places. Assuming these quantities to be equal, it appears 

 that the index at the Observatory moves nearly twice as fast as that 

 at the Society's house. The equality of the wind at Cambridge and 

 Edinburgh cannot so safely be assumed ; but if we proceed upon the 

 equality for March, as our only accessible basis, we shall find that the 

 index of the Society's Anemometer moves more than twice as fast as 

 that of the Edinburgh one. But I shall return to this comparison in 

 another form. 



In order to exhibit the general course of the winds at each place 

 I have adopted the following graphical method. 



Assuming, on a sheet of paper, the proper relative directions of the 

 points of the compass, I begin from a point and draw a line in the 

 direction of the first recorded wind, and of such a length as to represent 

 this wind in magnitude on a scale of equal parts. From the extremity 

 of this line, I draw another line representing in direction and magnitude 

 in like manner the second recorded wind; and from the extremity of 



