An examination of the Glen-mor-na-albin, or Great Glen of Scotland, stretching in a diagonal line across the island from Inverness to Fort William, has convinced me, that it has owed its origin to some convulsion of nature, and that the opening of this vast chasm, was the cause of the discharge of the water of the lakes, and of the change of the direction of the current of the rivers, which now run to the western, instead of to the eastern sea, as they seem to have done formerly. He conceives also, that the horizontal shelves of Lochaber, and this vast crack across the island, reflect a mutual light on each other, elucidating the history of both.

March 16th. Professor Leslie read an account of his new instrument called the Etherioscope; but as a full description has already been published, it is unnecessary to give any abstract of his paper at present.

At the same meeting, Dr. Brewster communicated to the Society a paper on a new theory of Double Refraction.

ART. XXI. On Street Illumination. By JOHN MIL

LINGTON, Esq.

AT a time when the lighting up of our streets is so much improved by the almost general adoption of coal gas, any observations on this head may be deemed superfluous; but the possession of a good light, affords no reason for the waste of it, which constantly occurs from the dark colour, and light absorbing nature of the covers which are at present made use of for street lamps. It was not a little amusing, before the present general introduction of gas lights, to observe the various expedients which were resorted to in the streets of London, to augment the scanty pittance of light which was allowed to the inhabitants by the penurious contractors for the supply of oil; and the confines of each parish could be clearly ascertained by its bull's eye lenses, dazzling the eyes of the passenger at every VOL. V.

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thirty yards, and then leaving him in almost total darkness; or by the various contorted reflectors twisted into almost every shape which imagination could suggest, though in most cases without enough of optical knowledge to know what their effect would be until tried for a season, when they were most frequently laid by to give way to new forms equally inefficacious. These were in a great measure rendered nugatory by the very excellent and ingenious lamp of Lord Cochrane, for which he obtained a patent, but which was afterward set aside by the decision of a court of law. These lamps have been for some time used in the parish of St. Anne, Soho, and St. John the Baptist, Savoy Precinct, and are decidedly the best street lamps for oil which are at present in use. Their principle depends upon constantly admitting a current of atmospheric air to play upon the burner through a tube, instead of inclosing the flame in a glass vase, having but one common opening at the top for the passage of the smoke outwards, and the entry of that air which is necessary to support combustion; and by covering the whole opening of the glass vase with a concave reflector of planished tin placed above the flame, which reflects all that light downwards which in all other cases is lost. The flame of these lamps is made rather larger than usual, which of course implies a greater consumption of oil; but in this his Lordship was guided by true philosophical reasoning, since it was accurately ascertained by Count Rumford, that if burning 228 grains of oil in a given time produced 100 degrees of light, as measured by his photometer, that 441 grains consumed in the same time would yield 600 degrees of light; while 560 grains produced 900 degrees: and thus a six-fold light was produced by less than a double quantity of oil; and by the further addition of little more than half the first quantity, the original light was increased in the proportion of nine to one; which prodigious increase Count Rumford accounted for* on the present generally received doctrine of flame, viz., that as the particles of which flame is

* See his Seventeenth Experimental Essay on Light,

composed are so far cooled as to be no longer red hot, they cease to be luminous, and consequently to be visible; the object in all cases of illumination, is therefore to preserve the heat of flame as long as possible, which will be accomplished by producing a larger fire from uniting the oil used in two lamps to be consumed in one wick in the same time, which, by the foregoing experiments, owing to the increase of heat, will produce light in the proportion of 6 to 1. This circumstance was fully proved in St. John's parish before named, when although but half the usual number of lamps were used in the streets, at least three times as much beneficial light was produced, as by the old method.

Although the reflectors adopted by Lord Cochrane are the most efficient which I have seen for producing that equal distribution, instead of concentration of light, which is so desirable in the streets of a town, yet they possess disadvantages which have not yet been overcome. They, in common with all the other reflectors I have seen, are made of planished or hammered tin, and so polished, that when new, they reflect a tolerably perfect image of the flame; but although tin, from its cheapness, is perhaps the best metal which can be used, still, notwithstanding they are better protected from smoke in Lord Cochrane's lamps than in many others, they are liable to oxidation or tarnish, by which they become inefficient; nor can they be expected to be kept in proper order by the parties to whose care they are entrusted, their numbers being great, and the time for attending to them very limited besides which, tin, from being thin, is liable to bruises and loss of its proper figure, and as it consists merely of iron plates thinly coated with the soft metal tin, this soon wears away by cleaning, when all power of reflection is lost. Besides this, a perfect reflecting surface is not necessary, nor indeed so good, in a street lamp, as one which, from not absorbing the light, throws it downwards without producing a focus or concentration of light sufficient to dazzle the eyes of the passengers. My attention being drawn to this subject about ten years ago, I was induced to try a number of experiments upon the powers

of different reflecting substances, and I found none of them so efficient for throwing down a plentiful and equally diffused light as the common glazed white earthen-ware, of which dinner plates and dishes are usually made, and of which any one may convince himself by simply holding a white plate 1 proin an inverted direction over a lighted lamp or candle. posed that flat circular plates or reflectors of this material should be made of a diameter equal to that of the opening of the glass vase containing the lamp, or the tin cover which is placed upon it, and that a hole of about two inches diameter should be left in the middle of the plate or reflector, or directly over the flame, wherever it might be, for the escape of the smoke, so that a plan of the reflector would appear like fig. 1, and a section through the middle of it like fig. 2, which also shows the situation of the flame. Fig. 1.

Fig. 2.

Upon mentioning this to some of the leading parties in a London parish, the only objections which were made were the fragile nature of the material, and its liability to become smoked, and lose its reflecting power; and I was requested to make some trials on these points, which I have since done, though I have never till now thought of making them public. The result was, that if a lamp is properly trimmed and adjusted, (i. e. the wick not placed too high, which never is the case with the street lamps which are contracted for,) no detrimental quantity of smoke is deposited in two or three nights burning, and when it does accumulate, it is instantly removed by a bit of tow or rag, with much less trouble than is necessary to keep tin reflectors in order. These reflectors may be

very conveniently fixed within the cover of the lamp so as to remove with it, by three or four bits of tin or wire soldered to it, and bent over the edge of the reflector, so that it has no chance of being broken except by a fall of the cover; and I find upon inquiry, that if such reflectors are obtained in a wholesale manner from the Staffordshire potteries, they can be furnished at from thee halfpence to twopence each. The flat under surface of fig. 2, will, I think, be the best for general use, but if a greater dispersion of light should be desirable, a reflector, of which fig. 4, is a central section, may be adopted, and on the contrary, where a concentration of light is wanted, as over door-ways, the concave form of fig. 3, the under surface of which is a portion of a hollow sphere, may be used.

Any of these reflectors, it will be seen, are applicable to Lord Cochrane's construction of an oil lamp; and since the general introduction of gas, which has in a great measure removed the objection of their becoming smoked, I think they will be found of utility to the public in all cases where light is to be cast downwards by reflection. Where the obstruction of an opaque reflector would be detrimental, and it is desirable to diffuse light in all directions, and at the same time to concentrate it on one particular object, or in one line, the use of a hollow cone of polished metal or earthenware, like a speaking trumpet, and having a similar opening at its apex, to the exterior of which the flame is to be applied, will be found very advantageous.