70 
PIaOF. TROUTOX AXD MR. XORLE OX THE MECHAXICAT. FORCES ACTIXO 
so that tan T Y 
cos“ ?v/tan a 
^ = XY = tan-i 
cos-' IL'\ 
) — a 
tan a/ 
(i-), 
Sin rj = sin QY = sin rQ sin iv = cos Tt sin ic 
sin 7V cos to 
\/ cos^ + tan” a 
(ii.) 
(ii.) The eftective couple due to the drift. 
Let sphere (hg. 5) represent the earth turning round its axis NS. 
Suppose the sun to he somewhere in 
the plane perpendicular to the paper 
and passing through the centre. 
Let OQ lie the direction of the drift. 
Then with same meaning as before for 
^ and 7], 
QNE=l7r-^^ NQ = 47r->7. 
Let P he a particular place on the 
eartli, which for London we take the 
latitude 51^°. Then NP = 38|° 
Let PCI he pdane of suspended con¬ 
denser jiassing through centre of earth, 
and making angle (j) with the meridian 
CPN = <l>. 
Call WNP, tlie time after G a.m., 6. 
Then PNQ = tt - d - (Itt - ^) = *77 - d + e 
Let OF he perpendicular to OP in the plane of the condenser. 
With P as pole draw a great circle arc FIT, meeting PQ produced at H. 
Then FH = /x, VQ = X. (See fig. 3.) 
Let rt lie the velocity of the drift along OQ. 
The component along OH = ^tsin X. 
So we have finally for the couple tending to rotate the condenser the value 
N {'ll/v)'^ sin” X sin 2/x. 
Tf it he possible, let us select the time of day so that X = 90 . 
Now 
cos PQ = cos NP cos NQ -p sin NP sin NQ cos PNQ 
cosX = sin 511 siim? + cos 5ll cos 77 sin (d — |).(m.). 
•sin {i — 0) — tan 51* tan p. 
N 
Fig. 5. 
When X = 90°, we have 
