Considering that, owing to the southerly position of Bombay, 
the type in the declination range differs considerably from 
that in our own latitude, the agreement is satisfactory, and, 
so far, tends to disprove the existence of vertical currents 
through the earth’s surface. 
The observations taken at Lisbon and Hobarton show an 
equally good agreement, those at St. Helena and the Cape 
of Good Hope less so; but in these two latter places, the 
observations taken at different months show a considerable 
difference of behaviour. 
We may now attempt another step, and try to gain an 
idea, however imperfect, as to the direction and intensity of 
the currents which may produce the diurnal variation. The 
variation in westerly force increases with the latitude, and 
we shall not go far wrong in taking it as a first approxima- 
tion proportional to the sine of the latitude. I write there- 
fore, with an arbitrary unit of force— 
Y = cosw cos(^ + X) 
where t is reckoned in arc from two o’clock Greenwich 
time. 
It is well known that in the expression for Y there is an 
important term having a double period each day, but it is 
not my intention to enter into any details at present, and 
we may see what we get with the above expression. 
c^Ysinw 
d\ 
Applying the equation 
and we obtain X = cos2?< sin(^ + X). 
The important point here is the factor cos2e^ which changes 
sign for a latitude of 45°. If our equation is approximately 
right the northerly force ought to be a maximum in the 
morning, a minimum in the afternoon in the equatorial 
regions, where cos2u is negative ; while in latitudes above 
45° the minimum ought to take place in the morning. This 
is exactly what happens, with the exception that the change 
seems to take place in latitudes smaller than 45° At Bom- 
