al
fertilisation of trees on the wholesale scale and for a purpose such
as this it is necessary that the trees to be operated upon shall not
be open to the outside atmosphere, but that the pollen dust, with
which the air inside the tent is to be laden, shall be strictly
confined during a stated period of time. Those methods of
fertilisation, with which the flower-gardener has in recent years
worked such wonders, can undoubtedly be utilised for many objects
besides those of the variation of form and hue in ornamental plants.

CHAPTER VIII.

MINING.

Exploratory telegraphy seems likely to claim a position in the
twentieth century economics of mining, its particular role being to
aid in the determination of the "strike" of mineral-bearing lodes. One
main reason for this conclusion consists in the fact that the
formations which carry metalliferous ores are nearly always more moist
than the surrounding country, and are therefore better conductors of
the electrical current. Indeed there is good ground for the belief
that this moistness of the fissures and lodes in which metals chiefly
occur has been in part the original cause of the deposition of those
metals from their aqueous solutions percolating along the routes in
which gravitation carries them. In the volumes of _Nature_ for 1890
and 1891 will be found communications in which the present writer has
set forth some of the arguments tending to strengthen the hypothesis
that earth-currents of electricity exercise an appreciable influence
in determining the occurrence of gold and silver, and that they have
probably been to some extent instrumental in settling the distribution
of other metals.

The existence of currents of electricity passing through the earth's
crust and on its surface along the lines of least resistance has long
been an established fact. Experiments conducted at Harvard, U.S.A., by
Professor Trowbridge have proved beyond a doubt that, by means of such
delicate apparatus as the telephone and microphone, it is possible for
the observer to state in which direction, from a given point, the best
line of conductivity runs. Under certain conditions the return current
is so materially facilitated when brought along the line of a
watercourse or a moist patch of the earth's crust, that the words
heard through a telephone are distinctly more audible than they are at
a similar distance when there i