HOW TO STUDY METEOROLOGY. 
151 
amount of cloud visible, the scale being 0 — 10. The 0 repre- 
senting a clear sky, and 10 a sky wholly overcast. 
Wind. — Observations on the Wind cannot in general be car- 
ried out by amateurs in any very complete manner. Eye obser- 
vations are rather unsatisfactory, and serviceable mechanical 
apparatus is necessarily complicated and expensive. For these 
reasons it will be sufficient to make this a short section. 
A wind- vane can be procured from any good ironmonger ; the 
erection of it, however, requires care. The site must be very 
open, whether the vane be mounted on the top of a pole rising 
from the ground or on one rising from a building. It must, of 
course, be correctly “ orientated.” This must be done by com- 
pass, allowance being made for the deviation; and as the whole 
operation may be a troublesome one, the workmen employed 
must be closely watched. 
Contrivances for registering information as to the wind are 
called anenometers. Eobinson’s is recommended by Sir H. 
James, as “ simple in its construction, and is not liable to get 
out of order, whilst it registers the velocity of the wind at any 
moment, or the current of air passing the station during the 
hours between the periods of observation. 
“ It consists (Fig. 5) of arms, at the end of which there are four 
light hemispherical hollow cups, which, as Dr. Eobinson has 
demonstrated, i*evolve with one-third of the velocity of the cur- 
rent of wind acting on them. On the vertical axis which carries 
the arms, there is an endless screw, which communicates its 
velocity of rotation to a circular index. 
u This index has two graduated circles, the outer one of which 
is graduated for five miles from 0 to 500, and the inner one into 
five miles, each mile divided into furlongs. The movable hand, 
from the centre, indicates the number of miles of air in the 
current which has passed the station, as 5, 10, 15, whilst the 
final hand indicates the number of odd miles and furlongs (as 
3 m. 5 f.) at which the movable hand stands, beyond the five mile 
graduation. If, for example, the movable hand stands between 
15 and 20 on the outer circle, and the final hand indicates 
3 m. 5 f., the length of the current 18 m. 5 f. 
si The velocity of the wind at any particular moment may be 
found by observing the index before and after a certain interval 
of time, as one or five minutes, and then multiplying the rate 
by 60 or 12 to find the velocity in miles per hour.” 
The pressure in pounds per square inch can then be ascer- 
tained by reference to a table. A milled head-screw, at the 
back of the instrument, turns the movable index, which should 
be brought back to zero after an observation is registered. The 
instrument requires to be erected in some open situation, easily 
accessible to the observer, but freely exposed to the wind. 
VOL. vi. — NO. XXIII. N 
