“When the lights are coloured, the shadows are coloured also, and it is then far more difficult to judge accurately of their intensity. They may in that case be much better seen from the point x, as the black opaque body which is interposed between them renders the difference of colour less sensible to the eye.
“The opaque body M is a cylindric rod of iron, whose upper part is blackened in the flame of a lamp, in order to prevent the reflection which might interfere with the sharpness (netteté) of the shadows, and to make them more distinct when they are viewed from the point x.”[86]
[86] Those who feel a curiosity to look farther into this subject may consult Count Rumford’s elaborate paper in the Phil. Trans. for 1794, p. 67.
I shall make a few trifling additions to M. Peclet’s clear description of his excellent mode of measuring the intensity of lights. It is, of course, presumed throughout, that the centres of the flames should be on one level; and I have found it most convenient to place the lamps on small carriages with rollers, which are guided by means of fine strips of wood nailed along the table in the directions gM and fM, and carrying the divided scales of centimètres. This affords the means of making any slight change in the position of the lamps so easily, as entirely to avoid the disturbance of the flame which ensues from lifting the lamp and readjusting it in another position; and will, in practice, be found very convenient when many observations are to be made. I have already said that my own experience has satisfied me that, with the aid of a good opera-glass, the central observation of the two shadows, with the opaque rod between them, is by far the best, and conducts, at once, to a result which is confirmed by the observations of two assistants who watch the shadows at the same time on opposite sides of the table, and at equal distances from them. I have found it convenient in comparing lights, to cover the table with dull black linen cloth, and to surround it with curtains of the same material, hung from slender brackets, in such a manner as to leave space for the observer to move freely round the table within them. The curtains prevent reflection from the walls of the chamber in which the experiments may be conducted, and also lessen the disturbing effects of currents of air. When a comparison of the intensity, and not of the aggregate power of two flames, is to be made, it is necessary to adopt the precaution of inclosing the lights in opaque boxes, with slits of equal area in each, placed on the same level, and so arranged, in reference to the flames, as to be directly opposite the brightest portion of each. After what has been said, it will be almost needless to add that the quotient of the square of the greater observed distance divided by the lesser, is the ratio of the illuminating power of the two flames. The most convenient mode of registering observations, and that which is generally practised, is in the form of a Table like the following:—
| Trials. | Distance. | Squares of Distances. | Illuminating Power, or Quotient of Squares. | |||
|---|---|---|---|---|---|---|
| Lamp A. | Lamp B. | Lamp A. | Lamp B. | Lamp A. | Lamp B. | |
| 1 | 143 | 140 | 20,449 | 19,700 | 1·00 | 0·958 |
| 2 | 117 | 114 | 13,689 | 12,996 | 1·00 | 0·949 |
As a standard lamp by which to test others, I believe few will be found superior to the best Carcel lamp, which has a clockwork movement, and whose flame continues to increase in power for about four hours after it is lighted; after which it maintains its state permanently, until the supply of oil fails. This fact was verified by M. Peclet with the greatest care. “I took,” says he, “two similar lamps. They were lighted at the same time, and their relative intensities were measured. One was then extinguished, without touching the wick, and its clockwork movement was stopped. One hour afterwards, I set the clockwork in motion and relighted the lamp, but without touching the wick. It was found in the same state as at the first comparison, and I measured its intensity in reference to the first. Those experiments I repeated every hour, and these are the results which I obtained. The lamp which I call No. 1, is that which remained continually burning; No. 2, is that which was only lighted during the continuance of the (successive) observations.”
| Times of Observation. | Intensities. | ||
|---|---|---|---|
| Lamp, No. 1. | Lamp, No. 2. | ||
| H. | M. | ||
| 5 | 30 | 100 | 100 |
| 6 | 30 | 103 | 100 |
| 7 | 30 | 106 | 100 |
| 8 | 30 | 110 | 100 |
| 9 | 30 | 117 | 100 |
| 10 | 30 | 117 | 100 |
| 11 | 30 | 117 | 100 |
| 12 | 30 | 117 | 100 |
This curious scale of increase in power, seems to be solely due to a peculiarity of the manner in which the lamp, that derives its supply of oil by clockwork, becomes heated; and the effect may be described as follows: The heating of the wicks, the chimney, and the oil in this burner, as in that of all other lamps, tends to increase the light; but, in an ordinary lamp, acting by a constant pressure, this maximum of heat is soon attained; whereas in the clockwork-lamp, into the burner of which the oil is thrown up by a pump, the whole of the oil in the cistern must reach its maximum temperature before the best effect of that lamp is produced. After this state has been reached, there is no disturbing influence at work, and the lamp burns steadily as long as the oil lasts.
I have myself tried what may naturally appear to be the most simple mode of obtaining an unvarying standard-light, by employing a gas-burner, supplied from a gasometer under a constant pressure; but I found it very difficult to obtain satisfactory proof of the constancy of the pressure; and in a large town, where there are many burners around one, their lighting or extinction is found to exercise a material influence in changing the condition of the flame. I must confess that I have always been disappointed in attempting to use a gas-flame as a standard of comparison.
There are various dangers on the shores of Britain, more especially at the entrance of the great estuaries of England and also in Ireland, whose position is such as to put them beyond the reach of regular lighthouses. Sand-banks which are too soft to sustain a solid structure, and have too deep water on them to admit of the erection of screw-pile lighthouses, are often the sites for Floating Lights. mooring light-vessels, to guide the mariner into the entrance of some estuary, or enable him to thread his way through the mazes of gats and channels, which, even during the daytime, baffle the mariner, who sees no natural object on the low sandy shores of the neighbouring coast to help him to guess at his true position. The first Light-vessel moored on the coast of Great Britain, was that at the Nore in 1734. There are now no fewer than 26 floating lights on the coast of England.