1875] WRITINGS OF JOSEPH HENRY. 509 



In regard to the relative photometric power of lamps of 

 the same order charged with mineral and with lard oil, all 

 the experiments we have yet made on this point tend to'the 

 conclusion that in smaller lamps with the more volatile oils 

 a greater photometric power is obtained than with the same 

 lamp when charged with lard oil; but with the larger lamps 

 the reverse is the case, the lard oil burned in these lamps 

 giving greater power than the mineral oil. 



An unexpected difficulty arose in the course of the inves- 

 tigations for the introduction of mineral oils, on account of 

 the form of the flame. While a lamp with a constricted 

 chimney, like that used in the German student-lamp, gave 

 the greatest photometrical power, it was found that the shape 

 of the flame did not correspond with the arrangement of the 

 lens apparatus, a large portion of the light being thrown up- 

 ward toward the sky and another toward the earth. It was 

 only after a series of trials with chimneys of different forms 

 and button-deflectors that a flame of the best shape was ob- 

 tained. To compare these flames in actual use, they were 

 placed in succession in a light-house, with a lens of the 

 fourth order, and the photometrical power determined at 

 different distances, from a mile to ten miles in extent, by 

 interposing between the eye and the light a series of thin 

 colored glasses, until the light was totally extinguished. 

 It was found in these experiments that some of the flames 

 which had an appearance of greater brilliancy near by, failed 

 to produce comparatively the same effect at a greater distance. 

 Having settled upon the form of the flame to be used in 

 lamps of the lower orders, arrangements have been made for 

 the introduction of mineral oils into all the stations in the 

 third district, at which lights of the fourth and smaller orders 

 are at present in use. The substitution of mineral for lard 

 oil however is a matter of no small difficulty, and requires 

 to be made with great precaution. An entire change in all 

 the lamps is required ; the several parts of the apparatus 

 which in the case of lard-oil lamps were united by soft sol- 

 der must now be joined with spelter. 

 The importance of this was evinced by an accident which 



