8 
HINTS TO TRAVELLERS. 
of a valley any difference in the luxuriance of vegetation according to 
exposure probably indicates the influence of rainfall as guided by the 
prevailing wind. So, too, the appearance of lines of drifted debris on 
the banks some distance from the edge of a lake or river may be taken 
as indications of the height to which the waters sometimes rise; and 
conversely the appearance of rows of trees in the middle of a wide shallow 
lake may indicate the line of a river which has temporarily flooded the 
surrounding meadows. Such observations have an important bearing on 
climate. 
The appearance and amount of dew are also to be recorded. The most 
important points to notice are the hour in the evening when the deposit 
commences, and the hour in the morning when the dew disappears. It 
should be noted also whether the deposit of dew is in the form of small 
globules standing apart on exposed surfaces, or if it is heavy enough 
to run together into drops and drip from vegetation to the ground. 
Thunderstorms and Hail .—The occurrence of thunderstorms should of 
course be noted, and here the hour of occurrence is of very great im¬ 
portance, for thunderstorms frequently show a marked diurnal period. 
The appearance of lightning without thunder should be recorded when 
it is observed, but this will naturally be almost always after sunset. 
Hailstorms usually accompany thunderstorms, and sometimes take the 
place of them. The occurrence of hail is most frequent in summer, and 
records of the size of hailstones are important. If possible they should, 
if of great size, be photographed along with some object of known size, 
and their structure described. It might at least be noticed whether they 
are hard and clear, like pure ice, or opaque like compacted snow, or 
made up of concentric layers of clear and opaque ice alternately. 
S'now .—Snow falls in all parts of the world, although in tropical or 
subtropical latitudes only at great elevations above sea-level. The 
actual limits of snowfall at sea-level are as yet imperfectly known, and 
any observations of snow showers in the neighbourhood of the tropics 
are of importance. It is essential in such a case to record also the 
approximate elevation of the land. On mountains in all latitudes the 
position of the snow-line should be noted at every opportunity. This 
is the line above which snow lies permanently all the year round, or 
below which snow does not completely melt in summer; and it is a 
climatic factor of some importance. It may be remarked, for instance, that 
