i06 O71 ihe deduction of a Mean Result from a [Jan, 



of the ordinates, and then of ibe abscissas of these points be found, the 

 former will be the ordinate and the latter the abscissa of the point, the 

 position of which is the mean of the positions of the given points. 

 Further, if the ordinate be divided b}^ the abscissa of the mean point, 

 or, which is the same thing, if the sum of the ordinates be divided by 

 the sum of the abscissas of the given points, the quotient will be the 

 tangent corresponding to the point in which a line, drawn from the 

 centre through the mean point, meets the circuuiference. Finally, the 

 direction denoted by the point thus found, is that which it is proposed 

 to regard as the mean of the given series of directions of the wind. 



The form of register adapted to this mode of determining the mean 

 direction of the wind, consists of two columns, one of which, for the 

 sake of distinction, may be marked x and the other y. In the former 

 of these the abscissa of the point from which the wind blows is to be 

 entered, and in the latter tlie ordinate. In order to obtain the mean 

 result of any series of observations, the sum of the terms in column y 

 is to be divided by the sum in column x ; the quotient gives the tangent 

 corresponding to tlie required point, and thence the point itself, by 

 mere inspection of the anemoscope. The particular quadrant to which 

 the tangent belongs is indicated by the signs of the two quantities 

 from which it is derived. 



Of the two modes of determining the mean direction of the wind 

 v;hich have been described, the latter seems to have the advantage in 

 the following particulars. 



1st. It is determinate, only one definite mean result being obtaina- 

 ble from a given set of observations. 



2d. It assigns as the m.ean direction of the wind, one which lies be- 

 tween, and in the same half of the compass with, its most prevalent 

 directions. 



3d. When there are two exactly opposite directions of the wind, 

 the two points which, in other cases, serve to determine the mean di- 

 rection, coincide, rendering the result indeterminale, as it ought to he. 



In the accompanying table, containing the evening observations of the 

 wind made at the Madras Observatory in August and September 1836,* 

 the different forms of register are contraste d. The series for Sepieni- 

 ber has been selected, for the purpose of shewing that Lambert's regis- 

 ter sometimes assigns, as the mean dii eclion, one the remotest possible 

 from the prevalent directions of the wind. 



♦ These observations are to be found in tlie "Madi as Journal of Science ond I.itorninrf, 

 Ko. 13. 



