The waste of effort and of wealth involved in planting trees and assiduously cultivating the soil for the growth of poor crops decimated by disease is the prime cause of the dearness of fruit. If, therefore, it be true that the fruit diet is one which is destined to greatly improve the average health of civilised mankind, it is obvious that the tree-doctor will act indirectly as the physician for human ailments. When this fact has been fully realised the public estimation in which economic entomology and kindred sciences are held will rise very appreciably, and the capital invested in complete apparatus for fighting disease in tree life will be enormously increased.
Very long tents, capable of covering not merely one tree each, but of including continuous rows stretching perhaps from end to end of a large orchard, will become practically essential for up-to-date fruit-culture. An elongated tent of this description, covering a row of trees, may be filled with fumes from a position at the end of the row, where a generating plant on a trolley may be situated. At the opposite end another trolley is stationed, and each movable vehicle carries an upright mast or trestle for the support of the strong cable which passes along the row over the tops of the trees and is stretched taut by suitable contrivances. Attached to this cable is a flexible tube containing a number of apertures and connected at the generating station with the small furnace or fumigating box from which the poisonous gases emanate.
Along the ground at each side of the row are stretched two thinner wires or cables which hold the long tent securely in position. The method of shifting from one row to another is very simple. Both trolleys are moved into their new positions at the two ends of a fresh row, the fastenings of the tent at the ground on the further side having been released, so that the flap of the tent on that side is dragged over the tops of the trees and may then be drawn over the top cable and down upon the other side. Seen from the end, the movements of the tent thus resemble those of a double-hinged trestle in the form of an inverted V which advances by having one leg flung over the other. For this arrangement of a fumigating tent it is best that the top cable should consist of a double wire, the fabric of the tent itself being gripped between the two wires, and a flexible tube being attached to each.
As progress is made from one row to another through the drawing of one flap over the other, it is obvious that the tent turns inside out at each step, and if only one cable and one tube were used, it would be difficult to avoid permitting the gas to escape into the outer air at one stage or another. But when the tubes are duplicated in the manner described, there is always one which is actually within the tent no matter what position the latter may be in. It is then only necessary that the connection with the generating apparatus at the end of the row should be made after each movement with the tube which is inside the tent. For very long rows of trees the top cable needs to be supported by intermediate trestles besides the uprights at the ends.
The gas and air-proof tent can be used for various other purposes besides those of killing pests on fruit trees. One of the regular tasks of the tree-doctor will be connected with the artificial 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 rôle 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.