1893.] 



A New Portable Miner s Safety-lamp. 



491 



chamber. The chamber was found to clear itself of gas in two 

 minutes when both top and bottom apertures were left open. With 

 the bottom aperture only opened, it stood for five minutes without 

 appreciable change occurring in the composition of its internal atmo- 

 sphere. 



Precautions were always taken, before commencing the measure- 

 ments of caps, to make the eye as sensitive as possible, more especially 

 when observing the paler caps. This was effected by remaining in a 

 dark room for some time. The interior of the test-chamber and the 

 back of the lamp-glass received a dead-black coat : and advantage 

 was gained by coating in the same way all those brass parts of the 

 lamp which could receive and reflect the light of the flame or cap. 

 Very feeble daylight was found to cause much more serious inter- 

 ference than full gaslight did, with the observation of the cap. 



A suggestion made by Professor H. B. Dixon was found of great 

 advantage when examining feeble caps ; the eye was directed on the 

 side of the lamp, instead of directly upon the cap ; the pale image 

 of the cap was thus formed upon a very sensitive part of the retina, 

 and the cap was much more readily seen than when it was observed 

 directly. 



All attempts to measure the heights of flame-caps by means of 

 graduated scales fixed within the lamp ended in failure, since the 

 light emitted by the cap near its summit was far too feeble to 

 illuminate the scale. The method of measurement ultimately 

 adopted, which has served for making many hundred readings, is 

 described below. 



An ordinary flat parallel-ruler was pressed against the window of 

 the chamber in a horizontal position (fig. 2), and supported a vertical 



Fia. 2. 



