Page:Popular Science Monthly Volume 89.djvu/19

 Popular Science Montlihj

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���inn ill miml)i'r with thost- of the miiie- tifld units. Hchind each .sf|uaro a lamp i^ mounted. As a hostile ship [)asse.s ihroutjli a mine-field the nearest micro- phones pick up the \il)rations ot her propellers and the corresponding l on the board glow. The luminous annunciator may be twenty or more miles distant from the microphones; it may be in Chicago and the m i n e - field in New York Harbor, if there were any military advantage in that great separation. It is always possible to follow the course of an intruding \'es- sel merely bj- watching the lights as they flare up and die out in the squares of the 1 u m i n o u 3 a n n u n c i a t o r . The lamps ac- tually \isualize the course tak- e n b >■ the vessel under observation . If she enters square 2 2 o f the field the lamp behind square 22 on the annuncia- tor boanl glows; as she

slips into square 2:1, of the mine-field, lamp 22 is extinguished and lamp 2.^ Hares up. The accompaiu"ing diagram will explain the general principle.

Mines are expensi\-e. To provide them with microphones and to wire the microphones to a luminous annunciator board adds to the cost of the installation. Suppose that it were possible to use fewer mines, in other words, to use rather large squares, and suppose that

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The mines, provided with microphones to hear the vibrations of ships which seek to enter a harbor, are arranged in numbered squares. A luminous indicator on shore, marked off into squares corresponding in num- ber with those of the mines is electrically connected with the microphones. Each mine-square is represented on the indicator by a lamp, which glows in its proper square on the luminous board as soon as a hos- tile ship enters and is heard. Thus it is possible to follow by the successive flaring up of lamps the course of a submarine or battleship threading its way thrQugh the mine-field and to explode the right mine

��it were possible \n rlelermine nol merely the |)articular s(|uare into which;i hostile \essel has found its way but the particular mine of that sf|uare nearest w hich it happens to be -wouUI not that soke the problem of cheapening the installation and heightening its effectixeness.-"

With this idea in mind I have connec- ted with the I u 111 i n o u s an n u ncia tor board what ma>' be <;dli-il a "|)recisif>n indicator," the n r pos e o f which is to show which mine is to be explrided in order to des- tro\- llu' inter- loper. A single precision iufji- calur scrvt's for all the mines; f i> r the wiring /is such that the precision indicator can be switched into the circuit of any mino- s(|uare at will. The details cannot bo re\'ealed at the I)rescnt time, because they are the subject of a patent application awaiting official action. It nui\- be stated, however, that the de\ ices emplo\ed accurately locate a \essel in a s(|uare by averaging the momentary,- respf)nsi\eness of the four microphones at the corners of the sc|uare. It is very much as if a pencil were attach- ed by four cords tfj as man\' (lulling devices, the pull on each cord coming from a different jxiint of the rompa.ss and representing the intt'iisity of the sound heard in a microphone. Pulleil in alt

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