174 ri;or. teot^tox and me. noble on the mechanical foeces acting 
Whence, taking' PD = 90°, 
0 = cos 38^ cos ND + sin 38^ sin ND cos (vi -{■ tt — 0 — a), 
so that 
cos {6 + a — m) = cot 38|- cot ND.(v.). 
ND is determined by the time of year from equations (iii.) and (iv.), so equation (v.) 
qives us the value of 0, the hour when the resultant drift is perpendicular to the axis 
of suspension. 
Tlie azimuth of the drift NPD is given by 
sin NPD = sin {9 a — m) sin ND, 
which reduces to 
cos NPD = cos ND cosec 38|-°.(vi.l. 
The Best Conditions when the Drift can never he entirely TIorContal in this Latitude. 
We have when PD is any angle 
cos PD = cos 38|- cos ND + sin 38^ sin ND cos [m rr — 6 — a). 
When PD exceeds 90°, we must choose the time of day so that PD is a minimum. 
By dilfereiitiation we obtain the condition 
0 ■= m TT — a. 
In a similar manner the azimuth of this component (viz., the one at right angles to 
DP) is given by 
• AT-n-Tk • /iQ I \ sin ND , 
sin NPD = sin (6' -P a — m) = 0, 
sin PD 
so that NPD = 180°. 
If ND = |-7r + 384 + e ; then the component utilised is, of course, given by 
resultant X cos e. In the end table the component is given. It is also shown in hg. 9. 
If we call the magnitude of the resultant drift R, then 
E,3 = 15^ + 19® -h 2 X 15 X 19 cos V'Q, 
so that R = 34 cos (^V'Q) approximately. 
Now cos V'Q = — sin 38^ sin rj + cos 384 cos rj cos V'NQ, 
V'NQ = PNQ - PNW = = 477 ), 
cos V'Q = — sin 38^ sin rj + cos 384 cos y sin + a), 
cos 4^^Q = v /4 (I + cos V'Q) = \/’5 ~ • 3 ] 1 sin y + ’391 cos y sin (4 + a). 
Tlierefore 
R — Si s/ '0 — ‘311 sin y -f ’391 cos y sin (4 + o) • . . • (vii.). 
