462 H. A. Hazen — Prevailing Wind Direction. 



under discussion. The simplest method of application would 

 be to prepare a form in which to enter the observations in an 

 order most suited for computation. Strict attention must be 

 paid throughout to the algebraic signs. To those who are un- 

 accustomed to algebraic processes the following rules will be 

 well worth remembering. For algebraic addition — if the 

 signs of the quantities are alike add them ; if unlike subtract 

 them and give the result the sign of the larger quantity, e. g. 

 3+4=7, —16+4= — 12. For subtraction — change the sign of 

 the second quantity and proceed as in addition, e. g. (+3)— 

 (+4)=-l, (-16)-(+4) = -20. The form to be used is as 

 follows : 



b 

 w 



c 



N 



d 



S 



e 

 NE 



/ 



sw 



9 



SE 



h 



i 

 NE-SW 



k 



SE-Nff 



I 

 iX 



cos 45° 



m 

 kx 



cos 45° 



n 

 E-W 







l+ni 



P 1 Q 



N-S \l-m 



r 

 0+n 



S 1 



p+g\ r-^s 



12 



21 



26 



13 



9 







10 



4 



—10 



2-8 



—7-1 



—10 



—4-3 



-5-01 9-9 



—14-3 



4-9 1 19 



Direc- 

 tion. 



N7PW 



Fill in the columns a to h with the number of times the wind 

 has blown from the eight points of the compass. The remain- 

 ing computations are indicated at the head of each column. 

 The values in (l) and (m), i and k, multiplied by cos 45°, may 

 be taken from Table I. Having determined r and s, we have 

 reduced the winds from the eight points to two at right angles 

 to each other, or we have a right-angled triangle in which we 

 have given the base and perpendicular, the tangent of the true 

 angle of the wind being the one divided by the other. Winds 

 from 1ST and E are regarded as positive -f-. The quadrant in 



which the angle lies is obtained as follows 



- r = NW: +!\ 



+s — s 



SE : ± r 



+ S 



NE 



— 5 



SW. In writing the angle, N. or S. 



should always be placed first. The value of the angle may be 

 immediately taken out of the table "Values of A" when we 

 have r and s given. If either or both of these be greater than 

 160, divide both by such a number as will make the larger, 

 160 or less. Find the smaller number in the top horizontal 

 row of figures, the other in the vertical row at the left. The 

 number at the intersection of the two will be the angle sought, 

 unless s is smaller than r. If so, subtract the number from 90°. 

 In the example given, r=— 14*3; 5=4*9 at the intersection of 

 49 and 145 we find 19°; since s is the smaller we must sub- 

 tract 19° from 90° and we have — =N 71° W. It will be 



+ 5 



readily seen that in entering the final table we may ignore 

 the decimal points, although with a good deal more trouble 

 we could have gotten the same result by entering with 4*9 



