NATURE 
54 
| May >0, 1886 
“ a fal 
climate, such as agriculture, &c. It shoulg. ftirce Or a 
this that wheth-- vs a cerect of heat, the one or the 
ouer 
should, thanks to the winds, be communicated to a 
greater mass of air, and be less sensible in the stratum 
close to the ground. The thermic properties of other sur- 
faces are more immediately available in the lower stratum, 
and consequently, from the practical point of view, exert 
a greater influence on the temperature of the earth and of 
its immediate vicinity. 
“Tf, then, we confine our consideration to that which 
from the practical point of view is perhaps the most im- 
portant, the influence of forests on the state of tempe- 
rature in the stratum in which man generally lives, in so 
far as this can be determined in the ordinary way by ther- 
mometers, I think that our reply for this country (Sweden) 
will be less uncertain, and it is as follows :— 
“In the districts of our country which are open and are 
cultivated, during the annual interval of cultivation, a 
forest lowers the temperature of air and soil during 
evenings and clear nights, restricting the period of daily 
insolation, and thereby checks vegetation. 
“The other influences of forests on temperature are 
either so slight that they possess no practical importance, 
as, e.g, the moderation of cold in winter, or else are of 
such a character that they elude the ordinary mode of 
observation by thermometers. Among the effects of this 
nature we may mention the well-known fact that forests 
afford shelter against cold and violent winds to vegetation 
which would suffer from these winds, or to objects whose 
temperature is higher than that of the environment, as for 
instance the human body. It is in this last respect that 
the Swedish saying is true, namely, that ‘the forest is the 
poor man’s cloak.’ In certain cases it may also yield 
protection against the cold air or fog which on cold nights 
comes from districts in the vicinity which are visited by 
frost. The advantages on the score of temperature 
derivable from the forest may therefore be considered to 
resemble that obtainable from a wall, a palisade, a hedge, 
or any object of that nature. 
““On the one hand a forest, where it is close at hand, 
offers mechanical protection against cold and violent 
winds. On the other hand, it does injury either by re- 
taining the solar heat required by crops, or by lowering 
the temperature of the soil during clear nights, and thus 
favouring the development of hoar-frosts. At a distance 
forests have no sensible influence on the climate of 
Sweden. 
“Tf we wish to put these results to a practical applica- 
tion, it is impossible to say in general whether one should, 
or even could, clear the forest without injuring agriculture. 
But it appears that as regards the temperature, if we dis- 
regard the utility of forests in other directions, we might 
make extensive clearances without any prejudice to agri- 
culture. It is certainly not a mistake to say that our best 
cultivated districts are the freest from wood, nor is it a 
mere chance that the harvests are, on the whole, more 
sure in the open country than in the forest. In the event 
of a bad harvest it is, as I well know, the wooded districts 
which have suffered most. At the same time I must at 
once admit that these provinces are also influenced by 
other powerful physical factors, possibly even more active 
than forests, such as an elevated situation, a bad soil, the 
presence of swamps, &c. But nevertheless it appears to 
ine, after all that has been said in the preceding pages, that 
the forest has some bearing on the subject. 
“At the present day, the words spoken 130 years ago 
by Pastor P. Hégstrém, and at that time member of the 
Swedish Academy, are very generally applicable, inasmuch 
as it has been found that cultivation can to a great extent 
remove from a district its tendency to hoar-frost; this 
same result has frequently been obtained by draining or 
by clearing the forests, particularly those of deciduous 
timber, where the fogs, especially those which bring on 
, -pp~ar wo Nave their origin and their aliment. On 
the contrary, a pine forest is an excellent shelter against 
cold, especially when it can stand between the country 
and marshes or surrounding districts where the cold has 
its rise. If, however, the forest interferes with sunshine 
and with wind, it should be cleared. It results, therefore, 
that while in some districts the clearing of a forest has 
been beneficial in averting hoar-frost, in others the result 
has been directly the opposite.” 
RESULTS DEDUCED FROM THE MEASURES 
Of TERRESTRIAL MAGNETIC FORCE IN 
THE HORIZONTAL PLANE, AT THE ROVAL 
OBSERVATORY, GREENWICH, FROM 1841 
TO 1876 
IR GEORGE AIRY has recently published a valu- 
»“ able and extensive series of diagrams representing 
the diurnal changes in the magnetic forces in the hori- 
zontal plane at Greenwich between 1841 and 1876. In 
an introduction, the ex-Astronomer-Royal gives a short 
statement of the circumstances under which the magnetic 
work was undertaken at Greenwich, and the various 
changes which have taken place. With regard to the 
curves here brought together he writes as follows :— 
The form of the curves, and the position of the points 
on them corresponding to hours of solar time, leave 
no doubt that the diurnal inequality is due mainly—and, 
as far as I can judge, entirely—to the radiant heat of the 
sun ; and, it would seem, not to its heat on the earth 
generally, but to its heat on points of the earth not very 
distant from the magnets. In the hot months of the year 
the curve, though far from circular, surrounds the central 
point in a form which, as viewed from that central point, 
never crosses itself, and is, generally speaking, usually 
symmetrical with regard to E.and W. But in the cold 
months the space included in the curve is much smaller, 
in many cases probably not one-fifth of what it is in the 
summer months ; and the curve often crosses itself in the 
most bizarre fashion, with irregular loops at these 
crossings. In the summer months there is a certain 
degree of symmetry ; but here is, constantly, a preponder- 
ance on the west side, which leads me to imagine that 
the magnetic effect of the sun’s heat upon the sea is con- 
siderably greater than the effect on the land. 
To obtain some numerical basis for a report which, 
though undoubtedly imperfect, may convey some ideas 
on this wonderful subject, I have adopted the following 
course. I have confined myself to the months of June 
and July as probably the two hottest, and the months of 
December and January as probably the two coldest. In 
each of the curves applying to these months I have laid 
down a system of rectangular co-ordinates corresponding 
to the Greenwich astronomical meridian, and the line at 
right angles to the meridian (the geographical E. and W.). 
The extreme north ordinate and the extreme south 
ordinate were measured, and their sum taken, and inter- 
preted by a scale of measure formed in accordance with 
the theory of the instruments, and this interpretation 
forms the “range of meridian force in terms of the mean 
horizontal force.” In the same manner, the “range of 
transversal force” is measured. As the time of each two- 
hourly or hourly result is marked on the curve, there is 
no difficulty in fixing. approximately on the solar times 
corresponding to the extreme N. and S. values and the 
extreme E. and W. values mentioned above. These are 
all the elements of the magnetic record which are 
included in the table. 
? 
MOVEMENTS ON THE SUN’S SURFACE 
M A. BELOPOLSKY, of the Moscow Observatory, 
* states in Astronomische Nachrichten, No. 2722, 
some considerations of much interest regarding the solar 
