GAS.— Photometry.

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inclosed in a box, needs no precautions in the wayof excluding exterior light. The Letheby photo-meter, Plate II. (this Plate is a representation of acomplete photometric apparatus recently constructedby William Sugg, for the government of thedominion of Canada, to be used for official purposes,it differs only from that adopted by the MetropolitanGas Referees in being inclosed, and so forms itsown dark room), consists of an open bar of wood, fixedon suitable supports, and graduated from the centre toeach end in such a way that the divisions of the scaleindicate at once the number of times which the lightused for comparison exceeds in intensity the lightwhich is being tested. At each end of the bar aresuitable supports for the two lights, while on thebar itself is a small box mounted on wheels, so as tomove along the bar freely, and provided at the lowerpart with a small pointer, which, as the box is moved,passes over the graduations of the bar. It is bymeans of this box that the comparison between thelights is effected. In the middle of this box, andset vertically, is fixed a round paper disc, made semi-transparent by being saturated with melted sperma-ceti or paraffin. This disc is made up of threelayers of paper, the middle of which has a circle orstar cut from its centre, so that when the layers areplaced together the central part is more transparentthan the surrounding portion. An oblong hole oneach side of the box allows the light from each end ofthe photometer to fall on the disc, while an orifice infront enables the observer to examine each side of thedisc. In order to still further facilitate observation,two small vertical mirrors are arranged at the backof the box, at such an angle that the reflection ofeach side of the disc can be seen at the same timeby the observer stationed in front, and a comparisonthus made of their respective intensities. When thedisc is illuminated by a source of light on each side,and the reflections from the mirrors observed, itwill be seen that the central and most transparentportion of the disc appears brighter on the one sidethan on the other. If the box be then moved tothe right or to the left, as the case may be, a pointwill at length be reached at which the reflectionsfrom each side of the disc appear equal in intensity,and the transparent central portion of the disc willhave almost, if not totally, disappeared. The pointerattached to the box will then indicate the numberof times which the one light exceeds the other inintensity.

In the Evans closed photometer the disc is im-movably fixed in the centre of the photometer, thelight, the intensity of which is to be estimated, beingfixed at one end. The light or lights used forcomparison are movable, and in estimating the inten-sity of any source of light the comparison light isshifted away from or moved towards the central discuntil the reflections from each side appear equal.In order to facilitate working with the instrument,a handle enables the operator to move the compari-son light, at the same time that he observes thereflections from the disc. The instrument, like thebar photometer, is graduated, the scale, however,VOL. I.

commencing from each end and terminating at thecentre. It is generally supported on a table by fourlegs, two at each end, the space underneath thephotometer being devoted to accessory apparatus,such as meter, balance governor, pressure gauge,&c. Whichever form of photometer is used themethod of testing and of calculating the results isthe same. The source of light used for comparisonis generally that which has been adopted by thelegislature and specified in all Acts of Parliamentrelating to gas testing, viz., the standard spermcandle of six to the pound, and consuming as nearlyas may be 120 grains of sperm per hour. It isassumed that the amount of light given out is inexact proportion to the quantity of sperm consumed,so that any deviations from the normal consumptioncan be easily corrected by a simple calculation.The gas to be tested is consumed at the rate of 5feet per hour in a standard burner, which for com-mon 14 or 16 candle gas is Sugg’s London Argand,No. 1, and for cannel gas of about 20-candle poweris Sugg’s No. 7 steatite batswing. The Plate willgive a very good idea of the arrangement of appa-ratus needed to test the illuminating power of gas.The apparatus used in the London official testingstations chiefly differs from that shown in having anEvans closed photometer in place of the Lethebyform.

In making a test the first necessary proceedingsare to adjust the gas to the parliamentary consump-tion of 5 feet per hour, and to have the candlesburning clearly, with well-formed flames and glow-ing tips to the wicks. In adjusting the gas to therequired quantity the regulating cock and quadrant(e in the Plate) is used, the amount of gasactually passing to the burner being shown by themeter a. The dial of this meter is graduated,and is provided with two long hands, both of whichwork from the centre of the dial-plate; one of thesehands is in connection with the meter, while theother is actuated by clockwork, and performs anentire revolution in a minute. The construction ofthe meter is such, that the meter hand and minutetravel together at an equal rate when the properquantity of 5 cubic feet of gas per hour is beingpassed. The meter dial has also on its surface twosmall graduated circles, provided with indicatorhands, one circle being divided into 12 parts, andindicating the consumption of 1 cubic foot of gas,the other circle in connection with the meter clock,and divided into ten divisions of one minute each.The gas having been adjusted to the normal rateof consumption, and the candles burning well, themeter clock is stopped, and by means of the stringattached to the winding-up arrangement, the smallhand of the dial is brought to zero. The candlebalance, G, is then so adjusted that the weight slightlypreponderates on the candle side, and when thebalance turns over the meter clock is started, and a40-grain weight dropped into the balance pan. Photo-metric observations are then made every minute, andas the completion of the tenth minute approaches thecandles are watched. When the balance again turns130