BURIED SHELLS.

New Zealand Herald, Volume LIII, Issue 16121, 8 January 1916, Page 4 (Supplement)

BURIED SHELLS.

AN AGRICULTURAL DANGER. When the erstwhile battlefields of Europe are reclaimed for the peaceful purposes of agriculture, there is an ever-present risk of death or serious injury to both the farmers and their horses as the result of ploughshares coming in contact with buried shells that have failed to explode when fired. The danger from unexploded shells buried in fields 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 Rendus of the Academy. 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 ti»'e 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 trained ear easily distinguishes between the two.

In the. shell-detecting instrument the distances from tile coils to the object sought are much greater than those in preceding applications of the Hughes balance. Therefore, two coils of large diameter are employed ; the device being in reality two induction balances used as one. In the three instruments thus far constructed in France, the diameter of the coils is 70 centimetres 'about 28in). The winding of the primary circuit consists of twenty layers, while that of the secondary circuit has ten layers. These windings | are placed on wooden spools of a size not | unlike that of the frames of ordinary j sieves. The rigidity of the spools is imI proved by the use of two diametrical rej inforcing braces or crossrods. As is well I known to those familiar with the principle of the induction balance, no metal parts of any kind can enter into the construction of the device. Hearing the Shells. To explore the desired territory, the two coilseach of which comprises a prij mary and secondary winding—are placed |on two vertical sticks attached to the j ends of a horizontal piece of bamboo. An I assistant carries the device bv means of j a suitable handle, and walk's over the I field to be explored, holding the coils of j the balance a few centimeters above the i ground. The observer, wearing a teleI phone head piece and carrying about a | box which contains the battery, condenser, j vibrator, and regulating apparatus, follows jat a distance of several telt behind the j assistant; a flexible conducting cable being | employed to connect the exploring coils j with the apparatus of the observer. I Fragment* of shells or tin cans or boxes, j on or near the surface produce a sound ias intense as that made by a deeply - : buried shell, but it is easy to distinguish j between the two. A superficial object j exerts the greatest influences on the device j when it is near the edges of the exploring j coils. Accordingly, daring the passage above such an object, the observer hears I two consecutive reinforcements of the , sound. A projectile deeply buried, on the I contrary, exerts its maximum action when i it is under the centre of either -coil, and j gives only one reinforcement of the sound i during the passage of the instrument. .! Each coil of the instruments that have I been already constructed explores a strip j equal to its width ; therefore, two strips, J 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 one half acres! of I land. '