ce. Thus we have many accounts of a
gradual desiccation which seems to have been going on over a large
region in Central Asia during historical times. In northern Africa
certain ancient historical records have been taken by different writers
to indicate a general decrease of rainfall during the last 3000 or more
years. In his crossing of the Sahara between Algeria and the Niger, E.
F. Gautier found evidence of a former large population. A gradual
desiccation of the region is therefore believed to have taken place, but
to-day the equatorial rain belt seems to be again advancing farther
north, giving an increased rainfall. Farther south, several lakes have
been reported as decreasing in size, e.g. Chad and Victoria; and wells
and springs as running dry. In the Lake Chad district A. J. B. Chevalier
reports the discovery of vegetable and animal remains which indicate an
invasion of the Sudan by a Saharan climate. It is often held that a
steady decrease in rainfall has taken place over Greece, Syria and other
eastern Mediterranean lands, resulting in a gradual and inevitable
deterioration and decay of their people.

_What Meteorological Records show._--As concerns the popular impression
regarding change of climate, it is clear at the start that no definite
answer can be given on the basis of tradition or of general impression.
The only answer of real value must be based on the records of accurate
instruments, properly exposed and carefully read. When such instrumental
records are carefully examined, from the time when they were first
kept, which in a few cases goes back about 150 years, there is found no
good evidence of any progressive change in temperature, or in the amount
of rain and snow. Even when the most accurate instrumental records are
available, care must be taken to interpret them correctly. Thus, if a
rainfall or snowfall record of several years at some station indicates
an apparent increase or decrease in the amount of precipitation, it does
not necessarily follow that this means a permanent, progressive change
in climate, which is to continue indefinitely. It may simply mean that
there have been a few years of somewhat more precipitation, and that a
period of somewhat less precipitation is to follow.

_Value of Evidence concerning Changes of Climate._--The body of facts
which has been adduced as evidence of progressive changes of climate
within historical times is not yet sufficiently large and complete to
wa