and the Velocity of the Wind. 358 
In the case of the trade winds, a part of the polar cyclones, 
the inclination of the direction of the wind to the isobars at 
sea is about 45°, and this being at about the latitude of 20°, 
the value of this angle, by theory, should be very much greater 
than its value at sea at the parallel of 50°, which from Mr. 
Ley’s small value obtained for the coast stations is perhaps 
less than 10°, so that this also confirms the preceding theo 
. Having now learned something from both observation 
and theory with regard to the value of the theoretically un- 
nown angle ¢ contained in the relation expressed by (1), we 
gradients and velocities belonging to the two polar hemispherical 
cyclones. Observation shows that the barometric pressure is a 
maximum, and that, consequently, G vanishes about the paral- 
lel of 35° in the northern hemisphere, and a little nearer the 
uator in the southern hemisphere, and that there are calm 
belts, called the tropical calm belts, at these latitudes, except 
so far as they are occasionally disturbed by local cyclones. By 
the relation expressed by (1), if we put G=0, we must also 
ave v=Q, unless cos7 vanishes, which, we have seen, does not, 
except at the equator. There must, therefore, be a calm where 
G==0, and hence we have the tropical calm belts. At the 
equator we also have G=0O, and with this value (1) is satisfied 
with v=0, that is, with a calm belt, as observed at the equator ; 
but it is likewise satisfied by (2nsin/+u) vanishing at the 
equator, and hence v is arbitra 
15. The barometric pressures given by Professor Loomis 
(Meteorology, p. 18) indicates that at about the parallel of 64 
in the northern hemisphere, and about the parallel of 74° in 
ore, for reasons which have already been given, we must 
have calm belts there. The observations from which these 
ressures have been deduced perhaps did not sufficiently em- 
race all longitudes and all seasons to give the mean con- 
Stant pressures, unaffected by local circumstances and the sea- 
