ILLUMINATION 



881 



back. D is a slide-valve, which is opened to allow the oil to descend to the wick, 

 and is shut when the cistern is to be separated from the pipe of supply, at E, for the 

 purpose of recharging it with oil. The flame is modified, not by raising or lowering 

 the wick, as in common lamps, but by raising or lowering the bell-mouthed glass 

 chimney, which rests at its bottom on three points, and is moved by means of the 

 rack-work mechanism F. The concentric cylindric space A, A, B, B, contains a pint 

 imperial, and should be made entirely full before lighting the lamp ; so as to leave 

 no air in the cistern, which, by its expansion with the heat, would inevitably cause 

 an overflow of the oil. 



1206 



The following arrangement was adopted in these experiments for determining the 

 relative illumination of the different lights. Having trimmed, with every precaution, 

 my French mechanical lamp, and charged it with pure sperm-oil, I placed it upon an 

 oblong table, at a distance of 10 feet from a wall, on which a white sheet of paper was 

 stuck. One of Mr. Parker's hot-oil lamps, charged with a quantity of the same oil, 

 was placed upon the same table ; and each being made to burn with its maximum 

 brilliancy, short of smoking, the relative illumination of the two lamps was determined 

 by the well-known method of the comparison of shadows ; a wire a few inches long, 

 and of the thickness of a crow-quill, being found suitable for enabling the eye to 

 estimate very nicely the shade of the intercepted light. It was observed in numerous 

 trials, both by my own eyes and those of others, that when one of the lamps was 

 shifted half an inch nearer to or further from the paper screen, it caused a perceptible 

 difference in the tint of the shadow. Professor Wheatstone kindly enabled me to 

 verify the precision of the above method of shadows, by employing, in some of thq 

 experiments a photometer of his own invention, in which the relative brightness of 

 the two lights was determined by the relative brightness of the opposite sides of a 

 revolving silvered ball, illuminated by them. 



1. The mechanical lamp was furnished with a glass chimney 1'5 inch in diameter 

 at the base, and 1'2 at top ; the wide bottom part was 1-8 inch long, and the narrow 

 upper part 8 inches. When placed at a distance of 10 feet from the wall, its light 

 there may be estimated as the square of this number, or 100. In the first series of 

 experiments, when burning with its maximum flame, with occasional flickerings of 

 smoke, it emitted a light equal to that of 11 wax candles, and consumed 912 grains 

 of oil per hour. The sperm-oil was quite pure, having a specific gravity of 0'874 



VOL. II. 3 L 



