048 
NATURE 
[AUGUST 22, 1912 
same way if our sun is moving relatively to the 
centre of gravity of all the stars, the stars must on 
the average seem to move away from the point 
towards which the sun is travelling, whilst they must 
close in towards the antipodes. These two points are 
called the apex and antapex of the sun’s path. 
Now Herschel concluded that there was something 
systematic in the proper motions of the stars, and 
that there was a point in the constellation of Hercules 
from which the stars were on an average receding, 
and that similarly they were closing in towards the 
antipodal point. ‘The first of these is the sun’s apex 
and the second the antapex. ‘These conclusions were 
drawn from the motions of comparatively few stars, 
but the result has been confirmed subsequently from 
a large number. Moreover, we have now learned 
by means of the spectroscope that we are travelling 
towards Hercules at the rate of about sixteen miles 
a second. 
During these last few years this grand discovery of 
Herschel’s has gained a great extension at the hands 
of Kapteyn and of many others, and it has been 
proved that other systematic motions of the stars are 
discoverable. The time at my disposal will not permit 
me to pursue this subject further, but I may say that 
it now appears that if we could view the universe from 
the centre of gravity of the stars of the Milky Way, 
we should see a current of stars coming from a 
definite direction of space and penetrating our system. 
What a vista of discoveries do these ideas open up 
to the astronomer! Some centuries hence the sun’s 
apex may have shifted, and we may perhaps learn 
that the solar system is describing the arc of some 
colossal orbit. The drift or current of stars may also 
have begun to change its direction, and our descend- 
ants may have begun to make guesses as to its future 
course and as to its meaning. But whatever develop- 
ments the future may have in store, we should never 
forget that the foundation of these grand conceptions 
of the universe was laid by Herschel. Holden ends 
his ‘‘ Life of Herschel”? with words which may also 
serve as a fitting end to my lecture: ‘‘As a practical 
astronomer he remains without an equal. In pro- 
found philosophy he has few superiors. By a kindly 
chance he can be claimed as the citizen of no one 
country. In very truth his is one of the few names 
which belong to all the world.” 
RECENT ADVANCES IN AGRICULTURAL 
SCIENCE—THE FERTILITY OF THE 
SOIL. 
ROM an ordinary common-sense point of view the 
fertility of the soil is best defined as that property 
for which a man pays rent—the property which causes 
some land to let for 2/. or 31. an acre, whereas the 
adjoining land may be dear enough at tos. With the 
causes of this fertility I do not propose to deal at any 
great length this evening more than to indicate that 
it is the outcome of a very complex series of factors, 
among which we can enumerate the actual supply of 
plant food in the soil, its mechanical texture as con- 
ditioning the movements of water, and the particular 
micro-fauna and flora inhabiting the soil, for upon 
these lower organisms depends the facility with which 
the material contained in the soil will become available 
for the nutrition of the plant. For the purpose of the 
present argument it will be sufficient to fix our atten- 
tion upon the amount of nitrogen in the soil as the 
main factor determining fertility, because, in the first 
place, nitrogen is one of the necessary and most ex- 
pensive elements in the nutrition of the plant, and, 
1 A discourse delivered at the Royal Institution on Friday, May 24, by 
A. D. Hall, F.R.S. 
NO. 2234, VOL. 89] 
secondly, because its amount in the soil is subject to 
both gains and losses from causes which are more or 
less under the control of the farmer. The other essen- 
tial elements which the plant has to draw from the 
soil—for example, phosphoric acid and potash—are 
only subject to slight losses by solution in the drainage 
water, and cannot be added to except deliberately by 
the action of the farmer; but in the case of nitrogen 
we have, in addition to the small stock of combined 
nitrogen in the soil, the vast store of free gaseous 
nitrogen with which both soil and plant are in con- 
tact. We may take it as settled nowadays that the 
plant itself can make no use of nitrogen gas, but 
must draw combined nitrogen in one of its simpler 
forms, such as nitrates or ammonia, from the soil. 
Among the bacteria of the soil, however, there are 
two great groups, one of which is capable of breaking 
up compounds of nitrogen and setting free the element 
as gas, whereas the other can take free gaseous 
nitrogen from the atmosphere and bring it into a 
combined form. Which of these two groups will be 
more active depends upon the conditions prevailing in 
the soil, and goes far to determine both its current 
fertility and the length of time during which it will 
be capable of bearing crops. 
The question of the duration of the fertility of the 
land under continual cropping has excited much atten- 
tion of late, chiefly because the United States has 
begun to take alarm about the reduced production of 
some of its most fertile lands, as, for instance, the old 
prairie lands of the middle West—a reduced produc- 
tion which, amongst other causes, has helped to set in 
motion a stream of migrants from the United States 
to the newer lands of the Canadian North-West. In 
the development of agriculture three distinct stages 
may be observed, In the first place, we may have a 
process of pure exploitation of the initial resources 
of the soil, when the farmer is to all intents and 
purposes mining in its fertility. This is the process 
which, in the main, has been going on in America, 
and, indeed, in all the newer countries which have 
been opened up to agriculture during the last two 
centuries. Not all virgin soils are rich, and the 
system of cropping alternately with wheat or maize 
which prevails over so much of North America has 
reduced great areas of the land in the eastern States 
to such a poverty-stricken condition that it has been 
allowed to go derelict. In the great plains, however, 
where the first settler found four or five feet of blacl: 
soil, containing nearly half per cent. of nitrogen, the 
land has kept up its productivity almost unimpaired 
for nearly a century. If we suppose the black soil 
only extended to a depth of three feet, and contained 
three-tenths per cent. of nitrogen, both limited esti- 
mates, there would still be 30,000 Ib. of nitrogen per 
acre—that is to say, nitrogen enough for five hundred 
crops larger than the American farmer has been 
accustomed to win from that land—and yet in less 
than a century such soils are beginning to show signs 
of exhaustion. The farming of the kind just de- 
scribed is destructive; but in the older lands of the 
west of Europe, which have been under cultivation 
for something like a century, a conservative system 
has been devised which is capable of keeping up the 
productive power of the soil, though not, perhaps, to a 
very high pitch. Perhaps the best example of this 
may be seen in the Norfolk four-course rotation prior 
to the introduction of artificial fertilisers. In this 
svstem a turnip crop, which was either consumed on 
the ground or converted into manure, and so returned 
to the soil, was followed by barley in which clover 
was sown, and the clover, which also got back to the 
soil, was followed by wheat. The farming covenants 
prevented the sale of anything more than barley and 
wheat grain, and the meat that was produced by the 
