a lamp of two concentric wicks, consuming about 130 gallons of oil per annum.

5th. Lights of the fifth order, having a focal distance of 7.28 inches (18.5 cm.); and

6th. Lights of the sixth order, having an internal radius of 5.9 inches (15 cm.), and lighted by a lamp of one wick, or Argand burner, consuming 48 gallons of oil per annum.

These orders are not intended as distinctions, but are characteristic of the power and range of lights, which render them suitable for different localities on the coast, according to the distance at which they can be seen. This division, therefore, is analogous to that which separates the lights of the United Kingdom into sea-lights, secondary-lights, and harbour-lights, terms which are used to designate the power and position, and not the appearance, of the lights to which they are applied.

Each of the orders is susceptible of certain combinations, which produce various appearances and distinctions, such as-fixed; fixed, varied by flashes; revolving, with flashes once a minute; and revolving, with flashes every half-minute, &c.

It has been found that, in fixed lights, the French improved refracting apparatus produces, as the average effect of the combustion of the same quantity of oil, over the whole horizon, upwards of four times the amount of light that is obtained by the catoptric system of paraboloidal reflectors.

The Ventilation of Lighthouses.-The ventilation of the lanterns forms a most important element in the preservation of a good and efficient light. An illventilated lantern has its sides continually covered with the water of condensation which is produced by the

contact of the ascending current of heated air; and the glass thus obscured obstructs the passage of the rays and diminishes the power of the light.

Professor Faraday's System of Ventilation.—An important improvement in the ventilation of lighthouses was introduced by Professor Faraday into several of the lighthouses belonging to the Trinity House, and has since been adopted in all the dioptric lights belonging to the Commissioners of Northern Lighthouses. The following is a description of Professor Faraday's apparatus :-The ventilating pipe or chimney is a copper tube 4 inches in diameter, not, however, in one length; but divided into three or four pieces; the lower part of each of these pieces for about one and a half inch is opened out into a conical form, about five and a half inches in diameter at the lowest part. When the chimney is put together, the upper end of the bottom piece is inserted about half an inch into the cone of the next piece above, and fixed there by three ties or pins, so that the two pieces are firmly held together; but there is still plenty of air-way or entrance into the chimney between them. The same arrangement holds good with each succeeding piece. When the ventilating chimney is fixed in its place, it is adjusted so that the lamp-chimney enters about half an inch into the lower cone, and the top of the ventilating chimney enters into the cowl or head of the lantern.

With this arrangement it is found that the action of the ventilating flue is to carry up every portion of the products of combustion into the cowl; none passes by the cone apertures out of the flue into the air of the lantern, but a portion of the air passes from the lantern by these apertures into the flue, and so the lantern itself is in some degree ventilated.

The important use of these cone apertures is, that when a sudden gust or eddy of wind strikes into the cowl of the lantern, it should not have any effect in disturbing or altering the flame. It is found that the wind may blow suddenly in at the cowl, and the effect never reaches the lamp. The upper, or the second, or the third, or even the fourth portion of the ventilating flue might be entirely closed, yet without altering the flame. The cone junctions in no way interfere with the tube in carrying up all the products of combustion ; but if any downward current occurs, they dispose of the whole of it into the room without ever affecting the lamp. The ventilating flue is in fact a tube which, as regards the lamp, can carry everything up, but conveys nothing down.

The Advantage Commerce has derived from Fresnel's System. Of the many distinguished men of exalted genius who have extended the boundaries of human knowledge by their inventions, there are but few who have conferred greater benefit on commerce and maritime intercourse than Fresnel, who deserves to be ranked among those benefactors of the species who have consecrated their genius to the common good of mankind; and as long as commercial intercourse subsists between nations the solid advantages which his labours have produced will be felt and appreciated.

CHAPTER XVIII.

ON THE SPECTROSCOPE, AND SPECTRUM ANALYSIS.

IN Chap. IX. the method of analysing solar light into its component colours, and forming thereby the solar spectrum, the prismatic spectrum, or, as it is sometimes called, the Newtonian spectrum, is clearly shown. In Chap. XI. it is stated that the spectrum so formed is crossed by numerous dark lines parallel to each other, and perpendicular to the length of the spectrum. These dark lines were not observed by Newton, nor were they noticed by any other philosopher except Melville up to the time of Dr. Wollaston, who observed, in the year 1802, only two lines in the solar spectrum, which, according to him, is composed of but four colours, namely, red, yellowish green, blue, and violet.

In order that the reader may have a correct idea of the nature of Dr. Wollaston's observations, it may be well to give the description of his experiments in his own words, as published in the "Philosophical Transactions of the Royal Society of London" for 1802, pages 378-380. "If," Dr. Wollaston states, "a beam of daylight be admitted into a dark room by a crevice

th of an inch broad, and received by the eye at the distance of 10 or 12 feet through a prism of flint glass free from veins, held near the eye, the beam is seen to be separated into the four following colours onlyred, yellowish green, blue, and violet.

"The line A, Fig. 104, that bounds the red side of the spectrum, is somewhat confused, which seems in part owing to want of power in the eye to converge red

A

light. The line B, between red and green, in a certain position of the prism, is perfectly distinct; so also are D and E, the two limits of violet; but c, the limit of green and blue, is not so clearly marked as the rest, and there are also on each side of this limit other distinct dark lines, f and g, either of which, in an imperfect experiment, might be mistaken for the boundary of these colours.

Fig. 104.

B

C

D

מי

"The position of the prism in which the spectrum is most clearly divided is when the incident light makes about equal angles with two of its sides. I then found that the spaces A B, B C, CD, D E, occupied by them, were nearly as the numbers 16, 23, 36, 25.

"Since the proportions of these colours to each other have been supposed by Dr. Blair to vary according to the medium by which they are produced, I have compared with this appearance the coloured images caused by prismatic vessels containing substances supposed by him to differ most in this respect, such as strong but colourless nitric acid, rectified oil of turpentine, very pale oil of sassafras, and Canada balsam, also nearly colourless. With each of these I have found the same arrangement of these four colours, and, in similar positions of the prisms, as nearly as I could judge, the same proportions of them.

"But when the inclination of any prism is altered so as to increase the dispersion of the colours, the proportions of them to each other are then also changed, so that the spaces A c and c E, instead of being as before 39 to 61, may be found altered as far as 42 and 58.

"By candle-light a different set of appearances may be distinguished. When a very narrow line of the