DECEMBER 31, 1897. ] 
ent. At about Lat. 30° N. it was colder, but in 
other latitudes warmer, and at the equator 
about 6.5°C. warmer then than now. 
The decrease of air temperature with the in- 
crease of altitude above the sea-level is of very 
great importance in climatological considera- 
tions, and Hann has given a careful discussion 
of this phenomenon. 
The average decrease of temperature in 
mountain regions is 0.57°C. per 100 meters as- 
cent, and it seems to be quite uniform for both 
equatorial and arctic regions. For the very 
gradual ascents, such as long slopes, the tem- 
perature decrease is but about 0.40°C. per 100 
meters ascent; while for the free air it is prob- 
ably about 0.65°C. These values show con- 
siderable retardation of the dynamic change of 
temperature, which is about 1°C. per 100 
meters ascent. 
Among the peculiar winds none are more 
interesting than the hot, dry winds which occur 
in certain mountain regions, and which in Eu- 
rope are known as Foehn winds, and in the 
United States as Chinook winds. It was to 
have been expected that Dr. Hann would de- 
vote considerable space to these winds, because 
they are with him a pet subject; although his 
modesty has caused him to place his first contri- 
bution to its literature a long way down in the 
reference list, whereas it should have headed 
that bibliography. Probably no single class of 
winds have been so little understood as these 
Foehn winds; and after searching in all direc- 
tions for some reason for their existence, the 
simple theory that they are due to the dynamic 
heating of descending air masses which have a 
decreased relative humidity due to the increased 
temperature has sufficed to explain what was 
considered a most complex phenomenon. 
One very interesting problem in climatology 
is, to determine how the different elements, such 
as temperature, rainfall and cloudiness, vary 
with changes of latitude. This is shown by 
finding the average values of the elements for 
certain parallels of latitude throughout their 
whole length as they encircle the globe. 
The following little table gives for various 
latitudes the average temperature for the whole 
year in degrees Fahrenheit ; the average total 
annual depth of precipitation (which includes 
SCIENCE. 
1001 
both rainfall and snowfall) in inches; and the 
average annual amount of cloudiness measured 
on a scale of 0= a clear sky, and 100 =a sky 
entirely overcast with clouds. 
: a a cole) 
E Re ae BS 
4 a & BI S) 3S 
N. 80° 2° 14 — 
70 14° 14 59 
60 30° 19 61 
50 42° 23 58 
40 ile 21 49 
30 68° 24 42 
20 idilise 32 40 
N. 10 80° 76 50 
Equator 79° 77 58 
Sb 1a? 78° 67 57 
20 74° 30 48 
30 65° 26 46 
40 54° 37 56 
50 42° 46 66 
S. 60° ole [40] 75 
This table is also interesting from the fact 
that it indicates that we have meteorological 
observations 20° nearer the pole in the northern 
hemisphere than in the southern hemisphere. 
The last section of this volume, which treats of 
climatic changes, may be divided into two parts 
—that treating of changes in geologic times, and 
that of changes shown by the records of modern 
civilization. This last has already been given to 
readers of meteorological literature in Briick- 
ner’s Klimaschwankungen, and this same author- 
ity is freely quoted by Hann. But the outline 
of the various theories accounting for climatic 
changes in the distant past is a much needed sum- 
mary. Hann states briefly the main theories 
which have been promulgated and which depend 
on possible changes in the inclination of the eclip- 
tic, or in the eccentricity of the earth’s orbit. 
A rough computation convinces him that the 
effects of neither of these would account for 
the changes which must have taken place in 
the earth’s climate. Croll’s theory is given due 
prominence, but Hann finds it unsatisfactory. 
A theory developed by Luigi de Marchi makes 
a variation in the coefficient of transmission of 
solar radiation through our atmosphere the im- 
portant factor in these great climatic changes ; 
but no certain causes of increase or decrease in 
the amounts of water vapor or carbonic acid 
gas, which would mainly affect the values of 
this coefficient, have been pointed out as appli- 
