the fire-damp detecter, the tin shield, or any other description of lamp, is made as sensitive as the Davy, while its other advantages of resisting velocity, etc., are not in any way interfered with.  As a proof of this I may mention that a deputy of experience recently visited a working place to make his inspection.  He reported the stall to be free from gas, but when the manager and steward visited it with the detecter, which they applied to the roof (where it would have been difficult to put even a small Davy), it drew a sample of the atmosphere which, on being put to the test tube in the tin-shield lamp, at once showed the presence of fire-damp.  Out of twenty-eight tests in a mine working a long-wall face the Davy showed gas only eleven times, while the detecter showed it in every case.  The detecter, as will be perceived from the one exhibited, and the accompanying sectional drawing, consists simply of an oval-shaped India rubber ball, fitted with a mouthpiece.  The diameter is about 21/4 inches by 3 inches, its weight is two ounces, and it is so small that it can be carried without any inconvenience in the coat or even in the waistcoat pocket.  Its capacity is such that all the air within it may be expelled by the compression of one hand.

The mouthpiece is made to fit a tube in the bottom of the lamp, and when pressed against the India rubber ring on the ball-flange, a perfectly tight joint is made, which prevents the admission of any external air.  The tube in the bottom of the lamp is carried within a short distance of the height of the wick-holder.  It is covered at the upper end with gauze, besides being fitted with other thicknesses of gauze at certain distances within the tube; and if it be found desirable to further protect the flame against strong currents of air, a small valve may be placed at the inlet, as shown in the drawing.  This valve is made of sufficient weight to resist the force of a strong current, and is only lifted from its seat by the pressure of the hand on the mouthpiece.  It will be apparent from the small size and elasticity of the detecter that the test can easily be made with one hand, and when the ball is allowed to expand a vacuum is formed within it, and a sample of the atmosphere drawn from the breaks, cavities, or highest parts of the roof, or, of course, any portion of the mine.  When the sample is forced through the tube near the flame, gas if present at once reveals itself by the elongation of the flame in the usual way, at the same time giving an additional proof by burning with a blue flame on the top of the test tube.  If gas is not present, the distinction is easily seen by the flame keeping the same size, but burning with somewhat greater brightness, owing to the increased quality of oxygen forced upon it.