NATURE 
[15 
of shock, as determined from the dimensions of a body which it 
has overturned, is a quantity not obtainable from an earthquake 
diagram. It represents the effect of a sudden impulse. In an 
earthquake a body is overturned or shattered by an acceleration, 
J, Which is calculable for a body of definite dimensions. As 
obtained from an earthquake diagram / lies between ie and Ue 
. a 
_ where Vis the maximum yelocity, ¢ is the quarter period, and @ 
is the amplitude. The initial velocity given in the formula 
= ~ for horizontal projection used by Mallet as identical 
with /? in the first formula, are not identical quantities. The 
velocity calculated from the range of projection, when projection 
occurs, is identical with the maximum velocity as measured 
directly or calculated from a diagram. The values Ls are those 
a 
used by Prof. Milne in discussing the intensity of movement. 
‘The intensity of an earthquake at first decreases rapidly as the 
disturbance radiates, subsequently it decreases more slowly. A 
curve of intensities deduced from observations at a sufficient 
number of stations would furnish the means of approximately 
calculating an absolute value for the intensity of an earth- 
quake. 
Vertical Motion.—In soft ground vertical motion appears to 
be a free surface-wave which advances more rapidly than the 
horizontal component of motion. It commences with small, 
rapid vibrations, and ends with vibrations which are long and 
‘slow. High velocities of transit may be obtained by the ob- 
servation of this component of motion, and this is possibly an 
explanation of the preliminary tremors of an earthquake and the 
sound phenomenon. 
Velocity.—The velocity of transit decreases as a disturbance 
radiates ; near to an origin it varies with the intensity of the 
initial disturbance. In different kinds of ground, with different 
intensities of initial disturbance, and with different systems of 
observation, velocities lying between 630 feet and 200 feet per 
second were determined. Mr. Mallet determined a velocity in 
sand of 824 feet, and in granite of 1664 feet per second. Gen. 
_ Abbot observed velocities of S800 feet. ‘These various deter- 
_ miinations may all be strictly correct, the great difference between 
them being due partly to the nature of the rock, the intensity of 
_ the initial disturbance, and the kind of wave which was observed. 
In Prof. Milne’s experiments the vertical free surface wave had 
the quickest rate of transit, the normal being next, and the 
transverse motion being the slowest ; but the rate at which the 
"formal motion exceeds the transverse is not constant. As the 
amplitude and period of the normal motion approach in value 
those of the transverse motion, so do the velocities of transit of 
these motions approach each other. 
Tn stating the results, of which those given above are the 
principal, Prof. Milne refers to the particular experiments which 
_ support them, thus giving chapter and verse for his conclusions ; 
but he thinks that if the investigations were repeated by himself 
or by any other investigator, although much of what he has 
recorded would be substantiated, more accurate results might be 
_ obtained by taking advantage of his experience. Finally he 
_ gives examples of investigations which have yet to be undertaken, 
_ and as these are valuable for others working in the same field, 
we append them here :—(1) An accurate determination of the 
rate at which the velocity of transit decreases as a disturbance 
radiates from its origin ; (2) the relation between the velocity of 
transit and the intensity of the initial disturbance ; (3) the deter- 
mination of the rate at which the intensity of a disturbance de- 
creases as measured at different distances from the origin. This 
might perhaps lead to the construction of a curve of intensities 
_ from which the absolute intensity of the initial disturbance could 
be learnt ; (4) a more complete investigation of the vertical 
motion and of free surface waves; (5) an investigation of the 
“inward motion of shocks. In Prof. Milne’s experiments the 
movement of the ground from its neutral position zz towards the 
origin of the disturbance has been performed so rapidly that he 
has been unable with the instruments at his disposal to measure 
its velocity accurately. As this is probably the most destructive 
element of motion, he regards its investigation as exceedingly 
important ; (6) further investigations on the relationship between 
earthquake diagrams, and the overturning and projecting of 
various bodies ; (7) a repetition of these and of all other experi- 
ments, on different kinds of ground. 
~ 
THE INFLUENCE OF FORESTS ON 
CLIMATE 
THE third number of Petermann’s Mittheilungen for this year 
contains an article by Herr A. Woeikof on the influence of 
forests on climate. The commencement of a scientific investi- 
gation of this subject was made when the Bayarian forest 
meteorological stations were established, and when Prussia, 
Alsace-Lorraine, France, Switzerland, and Italy followed the 
example. Asa general rule it may be laid down that in the 
warm seasons, as between forests and places close at hand which 
are treeless (1) the temperatures of the earth and air are lower 
in the former, (2) their variations are less, (3) the relative 
humidity is greater. After examining observations as to evapora- 
tions, Herr Woeikof states that the influence of forests in dimin- 
ishing evaporation from water and the soil is so great that it 
cannot be accounted for alone by the lower temperature of the 
hot months, the greater humidity, or even by the shade. An 
important influence, which has hitherto been but little appre- 
ciated, is the protection from the wind afforded by the trees, and 
this the writer regards as more important than all the others 
together in reducing the degree of evaporation. With regard to 
the influence of forests on rain and snowfall, there is as yet only 
a single series of observations supplying comparative statistics, 
and extending over a sufficiently long period. These were taken 
in the neighbourhood of Nancy, and they show an important 
influence of forests in increasing the rainfall. It might appear 
that the effect of forests on rain in the climate of Central Europe 
in winter would be small, for the difference between the temper- 
ature and humidity of the forest and the open is very little, and 
the quantity of moisture in the atmosphere is small. But the 
observations show that it is at this time of the year that forests 
get much more rain. This the writer attributes to the clouds being 
lower, the resistance which the forest offers to the movement 
of the air, and to the moist west wind. Forests retain rain by 
the undergrowths of grass, moss, &c., much better than open 
ground, and let water off superficially only after a heavy rainfall ; 
the remainder filters upwards slowly, and much of it is used for 
the evaporation of the trees. Although forests, especially thick, 
luxuriant forests, cannot exist without certain supplies of moisture, 
yet it is the same to them when the supplies come, for they 
retain what they get and use it over a long period. One example 
of this is the Lenkoran forest on the west coast of the Caspian, 
where the vegetation is more luxuriant than in any other part of 
Europe, yet very little rain falls in summer, but the rainfall in 
autumn and winter i§ great. The water is stored up by the 
forest, and is used in evaporation during the heat of summer. 
Humidity of the atmosphere, however, is not inconsistent with 
a high temperature, as the Red Sea shows ; but in forests the 
humidity is due to the evaporation of the leaves—in other words, 
to a process by which heat is converted into work, and hence 
the coolness. Herr Woeikof then endeavours to ascertain the 
influence of forests on the climatic conditions of their neighbour- 
hoods in the western parts of the Old World, between the 38th 
and 52nd degrees N. latitude, the places selected being in all 
cases in the open. Thus for the 52nd degree eight stations are 
taken between Valentia in Ireland on the west and the Kirghiz 
steppes on the east ; for the 50th, Guernsey on the west, Semi- 
palatinsk on the east, and thirteen stations, and so on for each 
two degrees of latitude to 38°. The general result of the obser- 
vations in fifty-stations in six different degrees of latitude is that 
in Western Europe and Asia large forests have a great influence 
on the temperature of places near them, and that by their influ- 
ence the normal increase of temperature as we travel eastward 
from the Atlantic Ocean to the interior of the continent is not 
merely interrupted, but they give places far removed from the 
coast a cooler summer than those actually on the sea. A striking 
example of this is Bosnia. An examination of the statistics 
shows (1) that in Bosnia the summer is 2°°5 to 4°°5 cooler than 
in Herzegovina ; (2) even on the island of Lissa, in the full influ- 
ence of the Adriatic Sea, the summer temperature is more than 
a degree higher than that of Bosnia, which is separated by lofty 
mountain ranges from the sea. Bosnia owes this comparatively 
cool summer to its great forests, while Herzegovina is almost 
disafforested. To sum up: forests exercise an influence on 
climate which does not cease on their borders, but extends over 
a larger or smaller adjacent region according to the size, kind, 
and position of forest. Hence man by afforestation and dis- 
afforestation can modify the climate around him; but it is an 
extreme position to hold that by afforestation the waste places 
