In the above table, for the purpose of comparing the successive degrees of intensity, 100 represents the mean intensity of light during the first hour. The quantity of oil consumed per hour is given in grammes, of 151⁄2 grains each. The last column expresses the quantity of light produced with a like consumption of oil, which was in all cases 100 grammes. See [Candles].
The following table of M. Peclet is perhaps more instructive:—
| Nature of the light. | Inten- sity. | Consump- tion per hour in grammes. | Cost | Fat pro- ducing the same light. | Cost per hour. | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| per kilogr. | of light per hour. | ||||||||||||
| francs. | cents. | gram- mes. | cents. | ||||||||||
| 1. | Mechanical lamp | 100 | 42 | 1 | ·40 | 5 | ·8 | 42 | 5 | ·8 | |||
| 2. | Flat-wick mechan. do. | 12 | ·05 | 11 | 1 | ·40 | 1 | ·5 | 88 | 12 | ·3 | ||
| 3. | Hemispherical dome lamp | 31 | ·0 | 26 | ·714 | 1 | ·40 | 3 | ·7 | 86 | ·16 | 12 | ·0 |
| 4. | Sinumbra lamp | 85 | 43 | 1 | ·40 | 6 | ·0 | 50 | ·58 | 7 | ·0 | ||
| 5. | Do. with a lateral fountain or vase | 41 | 18 | 1 | ·40 | 2 | ·5 | 43 | ·90 | 6 | ·1 | ||
| 6. | Do. with a fountain above | 90 | 43 | 1 | ·40 | 6 | ·0 | 47 | ·77 | 6 | ·6 | ||
| 7. | Girard’s hydrostatic lamp | 63 | ·66 | 34 | ·71 | 1 | ·40 | 4 | ·8 | 54 | ·52 | 7 | ·6 |
| 8. | Thilorier’s or Parker’s do. | 107 | ·66 | 51 | ·143 | 1 | ·40 | 7 | ·1 | 47 | ·5 | 6 | ·6 |
| 9. | Candle, 6 in lb. | 10 | ·66 | 8 | ·51 | 1 | ·40 | 1 | ·2 | 70 | ·35 | 9 | ·8 |
| 10. | Do. 8 in do. | 8 | ·74 | 7 | ·51 | 1 | ·40 | 1 | ·0 | 85 | ·92 | 12 | ·0 |
| 11. | Do. 6 with smaller wick | 7 | ·50 | 7 | ·42 | 2 | ·40 | 1 | ·7 | 98 | ·93 | 23 | ·7 |
| 12. | Wax candle, 5 in lb. | 13 | ·61 | 8 | ·71 | 7 | ·60 | 5 | ·7 | 64 | ·04 | 48 | ·6 |
| 13. | Sperm candle, do. | 14 | ·40 | 8 | ·92 | 7 | ·60 | 5 | ·8 | 61 | ·94 | 47 | ·8 |
| 14. | Stearine candle, do. | 14 | ·30 | 9 | ·35 | 6 | ·00 | 5 | ·5 | 65 | ·24 | 37 | ·1 |
| 15. | Coal gas | 127 | 136 litres | 5 | ·0 | 107 litres | 3 | ·9 | |||||
| 16. | Oil gas | 127 | 136 do. | 5 | ·0 | 30 | 3 | ·9 | |||||
The light of the mechanical lamp is greatly over-rated relatively to that of gas. The cost of the former is at least 5 times greater than of the latter, in London.
LAMP OF DAVY consists of a common oil lamp, surmounted with a covered cylinder of wire gauze, for transmitting light to the miner without endangering the kindling of the atmosphere of fire-damp which may surround him; because carburetted hydrogen, in passing through the meshes of the cylindric cover, gets cooled by the conducting power of the metallic gauze, below the point of its accension.
The apertures in the gauze should not be more than 1-20th of an inch square. Since the fire-damp is not inflamed by ignited wire, the thickness of the wire is not of importance, but wire from 1-40th to 1-60th of an inch in diameter is the most convenient.
The cage or cylinder should be made by double joinings, the gauze being folded over in such a manner as to leave no apertures. When it is cylindrical, it should not be more than two inches in diameter; because in larger cylinders, the combustion of the fire-damp renders the top inconveniently hot; a double top is always a proper precaution, fixed 1⁄2 or 3⁄4 of an inch above the first top. See [fig. 614.]