METEOBOLOGt AND CLIMATOLOGY. 
5 
the wind was blowing, hence the root nsually points to the direction of 
the wind. Waterspouts are closely allied to whirlwinds, and in any of 
those phenomena of revolving columns of air it is of much theoretical 
importance to determine the direction of the whirl about the axis, i.e. } 
whether the rotation is in the direction of the hands of a watch or the 
opposite. The prevailing wind of a district may often be discovered by 
the slope of trees growing on open ground, or still better by the difference 
in the degree of wave erosion on small lakes. If the banks are of the 
same material all round, the side against which the prevailing wind 
drives the waves will always be the most worn away. 
Cloud and Sunshine .—It would be impossible to keep a record of the 
countless changes in the cloud-covering of an English sky, but in many 
parts of the world the absence or presence of cloud is a function of lati¬ 
tude, altitude, and season, of great stability, and worthy of being atten¬ 
tively studied. The amount of cloud is usually estimated as the number 
of tenths of the sky covered; but it is a very difficult thing indeed to 
compare a tenth of the visible sky near the horizon with a tenth near the 
zenith. There is no difficulty, however, in observing when the sky is 
completely overcast or quite free of cloud, and as a matter of convenience 
the belt round the horizon to the height of thirty degrees may be neglected, 
i.e., the lower third of the distance from the horizon to the zenith. Very 
often it will be found that clouds form and disappear at certain hours of 
the morning or evening, and it is useful to get exact information on the 
subject. 
Of more importance than the amount of cloud is its nature, elevation, 
and movement. Distinct species of cloud have been recognised for a 
long time, and from more recent studies it would appear that they owe 
their distinctive appearance to the altitude at which they float in the air. 
Meteorologists distinguish a number of classes and transitional forms of 
cloud; it is enough for the traveller to be able to recognise the most definite 
types, viz., Cirrus, Cumulus, Stratus, and Nimbus. Cirrus clouds are 
the small tufts or wisps of cloud which float very high in the atmosphere, 
and to which the popular name of “ mare’s tails ” is applied. The 
transitional form, Cirro-Cumulus , popularly known as “mackerel scales” 
or “ mackerel sky,” is equally easy to identify. Cumulus clouds are 
great woolly-looking heaps of cloud, the lower surface of which is often 
nearly horizontal, while above they well into an exuberant variety of 
