360 MR RANKINE'S OBSERVATIONS MADE WITH WHEWELL'S ANEMOMETER. 



pencil descends is proportional to the joint eifect of the velocity of the wind, and 

 the time during- which it blows from any one point of the compass ; but the ane- 

 mometer does not record what portion of the joint result each element has per- 

 formed. An increase of time or velocity would be indicated in the same manner, 

 but it is impossible to discover from the tracings of the pencil what share either 

 has had in the joint result ; the velocity may vary from interval to interval, and the 

 same current may continue for a longer or shorter time, but the instrument will 

 always give the sum of all the elements of the current, or, in other words, will 

 " integrate the velocity multiplied into the differential of the time." 



The accuracy of this result, however, rests on the assumption, that the velo- 

 city of revolution of the fly is proportional to the velocity of the wind ; this has 

 not as yet been ascertained, but it seems exceedingly probable that a very near 

 approximation to such a result is given. 



The importance of the kind of results given by this anemometer may be 

 estimated, if it is considered how imperfectly the phenomena of atmospheric 

 cmTents are observed, if the direction only be recorded, and that too by merely 

 reckoning the number of days that the wind blows from each point of the com- 

 pass. Such a method is quite fallacious, for, in point of effect, the gentle breeze 

 of one day is placed on a par with the storm of another. The general relations 

 which some suppose to exist between the mean annual du-ections at different 

 stations, must depend very materially on the quantity of fluid transfeiTed ; it is 

 plain, therefore, that unless instruments registering the/o?'c^ as well as the direc- 

 tion of the wind be employed, we may hope in vain to acquire sound data for 

 meteorological speculation. 



This anemometer was erected on the roof of the University, about the middle 

 of November 1837, and its indications recorded up to the 1st of April 1838, except 

 for a few days whilst it was undergoing repairs. The First Table, at the end of 

 this paper, shews the register as kept during the months of December, January, 

 February, and March. The readings are in inches, tenths, and hundredths, on 

 the scale. 



The method of obtaining the mean direction of the wind for any given time, 

 is to reduce the partial winds into their component parts N, E, S, W. The sum 

 of all the east winds taken, and subtracted from the west, gives the effective 

 west wind, and the sum of all the north winds taken, and subtracted from the 

 south, gives the effective south wind. By compounding the magnitude and pro- 

 portion of the two effective winds, you find the magnitude and direction of the 

 effective wind between west and south which belongs to the whole time. With 

 the view of obtaining the mean direction and magnitude for these four months, 

 the Second Table has been constructed : it presents the same observations reduced 

 from thirty-two, to the four cardinal, points, by means of certain multipliers, found 

 by considering each intermediate Point as the hypotenuse of a right-angled triangle, 



