882 ILLUMINATION 



compared with water at 1000. In a subsequent series of experiments, when its light 

 was less flickering, and equal only to that of 10 wax candles, it consumed only 815 

 grains, or 0'1164 of a pound per hour. If we multiply this number into the price of 

 the oil (8s. per gallon) per pound, lid., the product l'2084d. will represent the rela- 

 tive cost of this illumination, estimated at 100. 



2. The hot-oil lamp burns with a much steadier flame than the mechanical, which 

 must be ascribed in no small degree to the rounded slope of the bell-mouthed glass 

 chimney, whereby the air is brought progressively closer and closer into contact with 

 the outer surface of the flame, without being furiously dashed against it, as it is by the 

 rectangular shoulder of the common contracted chimney. When charged with sperm- 

 oil, and made to burn with its maximum flame, this lamp required to bo placed one 

 foot further from the screen than the mechanical lump, in order that its shadow should 

 have the same depth of tint. Hence, its relative illumination was, in that case, as the 

 square of 11 to the square of 10 ; or as 121 to 100. Yet its consumption of oil was 

 only 696 grains, or somewhat less than 01 of a Ib. per hour. Had its light been 

 reduced to 100, it would have consumed only 576 grains per hour, or '082 of a Ib. If 

 we multiply this number by lid., the product 0'902d. will represent the relative cost 

 of 100 of this illumination. 



3. The hot-oil lamp being charged with the southern whale-oil, of specific gravity 

 0*926, at 25. 6d. per gallon, or 3|rf. per Ib., when burning with its maximum flame, 

 required to be placed 9 feet and 1 inch from the screen to drop the same tint of shadow 

 upon it as the flames of the other two lamps did at 10 and 11 feet with the sperm-oil. 

 The square of 9 feet and 1 inch = 82 is the relative illumination of the hot-oil lamp 

 with the southern whale-oil. It consumed 780 grains, or O'lll of a pound per hour ; 

 but had it given 100 of light it would have consumed 911 grains, or 0'130 of a pound, 

 which number being multiplied by its price 3|d., the product 0'4875e?. will represent 

 the relative cost of 100 of this light. 



4. A hot-oil lamp charged with olive-oil of specific gravity 0'914, at 5s. 6d. per 

 gallon, or 1\d. per Ib. when burning with its maximum flame, required to be placed 

 at 9 feet 6 inches, to obtain the standard tint of shadow upon the screen. It con- 

 sumed 760 grains per hour. The square of 9 feet is 90, which is the relative inten- 

 sity of the light of this lamp. Had it emitted a light = 100, it would have consumed 

 840 grains, or 0'12 of a pound per hour which number multiplied by the price per 

 pound, gives the produce 0'9d. as the relative cost of 100 of this light. 



5. A hot-oil lamp charged with Price and Co.'s cocoa-nut oil (oleine), of specific 

 gravity 0'925, at 4s. 6d. per gallon, or 5$d. per Ib. had to be placed 9 feet from the 

 screen, and consumed 1,035 grains per hour. Had its light been 100 instead of 81 (9 2 ), 

 the consumption would have been 1,277 grains, or 0'182 of a pound per hour ! which 

 number multiplied by its price per pound, the product l'Q31d. will represent the cost 

 of 100 of this illumination. 



6. In comparing the common French annular lamp in general use with the me- 

 chanical lamp, it was found to give about one-half the light, and to consume two-thirds 

 of the oil of the mechanical lamp. 



7. Wax candles from some of the most eminent wax-chandlers of the metropolis 

 were next subjected to experiment ; and it is very remarkable that, whether they wnv 

 threes, fours, or sixes in the pound, each afforded very nearly the same quantity of 

 light, for each required to be placed at a distance of 3 feet from the screen to .-ittbnl 

 a shadow of the same tint as that dropped from the mechanical lamp, estimated at 100. 

 The consumption of a genuine wax candle, in still air, is, upon an average of many 

 experiments, 125 grains per hour, but as it affords only y^th of the light of tin- me- 

 chanical lamp, 11 times 125 = 1,375 grains, or 0'1064 of a pound, is the quantity that 

 would need to be consumed to produce a light equal to that of the said lamp. If we 

 multiply that number by the price of the candles per Ib. = 30d. the product = 5'892e?. 

 is the cost of 100 of illumination by wax. A wax candle, three in the pound (short), 

 is one inch in diameter, 12 inches in length, and contains 27 or 28 threads, each 

 about y^th of an inch in diameter. But the quality of the wick depends upon the capil- 

 larity of the cotton fibrils, which is said to be the greatest in the Turkey cotton, and 

 hence the wicks for the best wax candles are always made with cotton yarn imported 

 from the Levant. A wax candle, three in the pound (long), is ths of an inch in 

 diameter, 15 inches long, and has 26 threads iri its wick. A wax candle, six to the 

 pound, is 9 inches long, *ths of an inch in diameter, and has 22 threads in its wick. 

 The light of this candle may be reckoned to bo, at most, about yyth less than that of 

 the threes in the pound. A well-made short three burns with surprising regularity in 

 still air, being at the rate of an inch in an hour and a half, so that the whole candle 

 will last 18 hours. A long three will last as long, and a six about 9 hours. Sp. gr. 

 of wax = 0-960. 



8. A spermaceti candle, three in the pound, is y^ths of an inch in diameter, 15 inches 

 long, and has a plaited wick, instead of tho parallel threads of a wax candle. Tho 



