BURIED SHELLS.

AN AGRICULTURAL DANCER.

When &© erstwhile battlefields of Europe are reclaimed for the peaceful purposes ■of "agriculture,. ' there is an ever-presont risk of death or serious injury to both tho farmers and their horses as the result of ploughshares coming hi contact with buried shells that have failed to explode when fired.

The danger from unexplpded shells buried' in nelds is regarded as particularly'grave in certain parts of France. Recently,' an engineer was commissioned to devise a method for the discovery and subsequent removal of such shells. His report on the matter was laid before the French Academy of Sciences ; an account of his- findings ! appearing in the "Comptes Bendus" of the Academy. I The instrument that has been devised by the French for the detection of buried shells is an adaption of the Hughes induction balance. The original instrument was made by C Gutton, at the request of the Prefect of the Department of Meurthe-et-Moselle, and with it, the constructor was able to detect the presence of a small calibre shell at a depth of about 40 centimetres (nearly 16in.). The apparatus, which was thoroughly tested, is so sensitive that its - user can detect by the sounds in the head telephones the proximity of a mere scrap of shell on or near the surface of the ground, or even a tin can. The note sounded by

a shell fragment differs from that

caused by a buried shell, so that the i trained ear easily distinguishes between the two. In the shell-detecting instrument the distances from the coils to the object sought are much greater than those in preceding applications* of the Hughes balance. Therefore, two coils of large diameter axe employed; the device being in reality two induction balances used as one. In the three instruments thus far constructed in Prance, the diameter of the coils is 70 centimetres (about 28in.). The winding of the primary circuit consists of 20 layers, while that of the secondary circuit has 10 layers. These windings are placed on wooden spools of a size not unlike that of the frames of ordinary sieves. The rigidity of the spools is improved by the use of two diametrical reinforcing braces or crossbreds. As is well known to those familiar with the principle of the induction balance, no ' metal parta of any kind can enter into the construction of the device. To explore the desired territory, the two' coils—each' of which comprises a primary and secondary winding—are placed on two vertical sticks attached to the ends of a horizontal^ piece of bamboo. An assistant carries the device by means of a suitable handle, and walks over the, field to be explored, holding the coils of the balance a few centimeters above the

ground. Tho observer, wearing a telephone head piece and carrying about a box which contains the battery, condenser, vibrator, and regulating apparatus, follows at a distance of several feet behind . the - assistant- a flexible conducting cable being employed to connect the exploring coils with the apparatus of the observer. .Fragments of shells or tin cans or fooxes, on or near the surface produce a sound as intense as that made by a

deeply-buried shell, but it is easy ..to distinguish between the two. A superficial "object exerts the \ greatest in•fluences on the device when it is near the edges of tho exploring coils. Accordingly, during the passage above such an. object, the observer hears two consecutive reinforcements of the fiouiid. A projectile deeply buried, on the contrary, exerts its maximum ac-

tion when it is under the centre of

either coil, and gives only one reinforcement of the sound during the

passage of tho instrument. Each coil of the instruments that have hcen already constructed explores a strip equal to its width ; therefore, two strips, each 70 centimetres wide, are..examined during each passage. Consequently, it requires about three hours for two persons accustomed to the work to explore one hectare (about two and onohall' acres) of land.