The telephone, the newest and in some ways the most
striking of all applications of electricity, has already received one practical application of importance. It is a very difficult thing to keep the ventilation of a mine perfect. The men neglect it, and when the inspector is coming down, prepare the air for his visit. It has been found, however, that the anemometer or air-measurer can be so adjusted that after a few revolutions it releases a spring, which touches the vibrating-plate of the telephone, and at 'once, therefore, warns the engineer above that the anemometer has completed the series of circles. In other words, he hears in his office the rate at which air is moving in the mine 600 yards below him, and this without human intervention. If there is not enough air, the same telephone enables him instantaneously, without rising from his chair, to call attention to the neglect.
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