XXVI INTRODUCTION. 



which is equivalent to considering the wind to have the same mean velocity 

 for all directions. 



If directions are observed to sixteen points, half the number belonging 

 to each extra point should be added to the two octant pbints between which 

 it lies; for example, NNE = 6 should be separated into N = 3 and NE = 

 3; ESE = 4, into E = 2 and SE = 2. The result will be approximately 

 identical with that obtained by using the complete formula for sixteen 

 points. 



Table 40. Multiples of cos 45 ; form for computing the numerator and de- 

 nominator. 



Table 41 . Values of the mean direction (a) or its complement (90 — a). 



Table 40 gives products of cos 45 by numbers up to 209, together with 

 a form for the computation of the numerator and denominator, illustrated 

 by an example. The quadrant in which a lies is determined by the following 

 rule : 



When the numerator and denominator are positive, a lies between 

 N and E. 



When the numerator is positive and the denominator negative, a lies 

 between S and E. 



When the numerator and denominator are negative, a lies between 

 5 and W. 



When the numerator is negative and the denominator positive, a lies 

 between N and W. 



Table 41 l combines the use of a division table and a table of natural 

 tangents. It enables the computer, with the numerator and denominator 

 of Lambert's formula (computed from Table 40) as arguments, to take out 

 directly the mean wind direction a or its complement. 



The top argument consists of every fifth number from 10 to 200. 



The side argument is given for every unit from 1 to 50 and for every 

 two units from 50 to 150. Tabular values are given to the nearest whole 

 degree. 



Rule for using the table : 



Enter the table with the larger number (either numerator or denomi- 

 nator) as the top argument. 



If the denominator be larger than the numerator, the table gives a. 



If the denominator be smaller than the numerator, the table gives 

 90 — a. 



,.a is measured from the meridian in the quadrant determined by the 

 rule given with Table 40. 



1 From Hand-book of Meteorological Tables. By H. A. Hazen. Washington, 1888. A 

 corrected copy of the table was kindly furnished by the author. 



