of the Atmosphere. 423 
ment. A pair of flanges were attached to this disc in such a 
manner that, when the whole was at rest and the air free from 
motion, the flanges would be horizontal. With perfectly hori- 
zontal currents, the flanges would still continue in the same 
position, although the head of the vane would as usual move 
about in azimuth. But if a current happened to be inclined to 
the horizon, the flanges would be pressed upwards or downwards, 
showing the direction and amount of the inclination, precisely as 
the position of the head or tail of the ordinary vane shows the 
direction and inclination of a currerit with reference to the meri- 
dian. When we know the inclination of a given current to the 
horizon, we can readily estimate its absolute force from its hori- 
zontal force, as can be easily shown. 
3. Let the origin of coordinates be at the centre of the axis 
of the vertical disc ; yda will represent an element of the area 
of the flange. Let @ represent the angle of inclination of the 
flange, H the pressure exercised by the wind in a horizontal 
direction upon a square unit of surface, and V the vertical pres- 
sure exercised upon a similar unit. The eutire moment of the 
horizontal forces acting on the entire flange will be 
H i) sin Oxydz, 
and the moment of the vertical forces will be 
V {cos Oxydx. 
Both of these moments tend to cause a rotation of the disc, 
but in contrary directions : hence when the dise is in equilibrium 
they must be equal, and therefore, because @ is independent of 
zx and y, we shall have 
H sind=Veos0,V=Htan9; - + + + (1) 
and if we write F for the absolute force of the wind, we shall 
have 
PaB wedge. vow ok wp 
Hence it follows, that if we can observe the absolute direction of 
the wind, we can estimate its vertical force as well as its absolute 
intensity without any special instrument, using the results ob- 
tained by the existing anemometers which give the horizontal 
intensity. 
4, A wind-vane or anemoscope, capable of showing the abso- 
lute direction of an atmospherical current, having been con- 
structed in accordance with my directions, I proceeded to make 
some observations during the months of June, July, and August, 
1857. It was placed on the top of a strong mast, about twenty- 
six feet in height. The mast was fixed near the end of a large 
garden, far from buildings. As my first series of observations 
2F2 
