In Fig. 16 the three gasholders, 10, 5, 1 cubic feet, are shown on the right of the picture as cylindrical measures suspended over tanks of water.

Two cubic foot "bottles" are shown on the left of the picture, one on either side of the clock; each conical bottle being also suspended over a water tank.

Every gas meter inspector's testing instruments are required to be reverified once in every 10 years (Weights and Measures Act, 1889, section 15).

In the measurement of the illuminating power of gas Photo-rather a qualitative than a quantitative measurement-meters. the Gas Works Clauses Act, 34 & 35 Vict. c. 41, 1871, prescribes the present form of photometer to be used by gas companies in testing the power of gas as the "improved form of Bunsen's photometer known either as "Letheby's" open 60 inch photometer, or as "Evans' " enclosed 100 inch photometer, as provided with a proper meter, minute clock, governor, pressure gauge, and balance.

There is no standard photometer, but such instruments as the meter, balance, and pressure gauge, &c. are standardized. (See also Diblin's Photometry, "Journal of Gas Lighting," London 1889; Hartley's "Gas Analyst's Manual," Spon, London 1879, &c.).

A useful form of photometer is that introduced by Mr. G. Lowe, C.E., in 1864, which simply indicates the illuminating power of gas from the quantity required under specified conditions to maintain a jet of gas or a flame a given height-as 3 inches.

There is no standard of a "Burner" except in some cases of "Sugg's London Argand Burner," specified in local Acts, and where the Board of Trade are required to certify a model of such burner, then the stamp of the Standards Office is placed thereon.

In connection with the Argand burner Mr. John Methven, C.E., has introduced a form of "screen by

G

Weighing of air.

means of which certain uniform photometric results can be obtained in the use of a burner.

As to a standard of light, it may be pointed out that, so far back as the year 1849, a standard candle was recommended by Professor Graham, Dr. Leeson, and Mr. Cooper. The Metropolis Gas Act of 1860 further provided that the common gas supplied shall be "such as "to produce from an argand burner having 15 holes and a 7 inch chimney, consuming 5 cubic feet of gas an hour, a light equal in intensity to the light produced by "not less than 12 sperm candles of six to the pound, each burning 120 grains an hour."

The Gas Works Clauses Act, 1871, sections 28 and 32, prescribes the mode of testing the illuminating power of gas supplied by gas companies, and in London the Metropolis Gas Referees issue Regulations with reference thereto.

In a Report made by a Committee on the Standards of Light in 1895 and laid before Parliament, a new form of standard of light known as the "pentane " standard burner, has been proposed in place of the sperm candle.

The Imperial Pound, the unit of mass, has its true legal weight" in vacuô" and hence the weight of air displaced by it, and by any mass compared with it, has to be allowed for. It has been remarked by Dr. Angus Smith (“ Air and Rain") that when we are children, air is to us, nothing— a vessel of air is a vessel with nothing in it-and that early nat ons thought in the same way. It was a great day for the world when air was found to be something material, and to be capable of weighing down the scale of a balance.

The weight in grains of a cubic foot of air, when the atmospheric pressure is B, expressed in inches, and the

temperature t, in degrees Fahrenheit, may be taken as

In the measurement of any given volume of air we have to allow, not only for variations of atmospheric pressure (B.) and of temperature (t.), but also for the variable amount of moisture contained in the air, and for the amount of carbonic acid. The weights above given refer to the air of rooms artificially warmed and illuminated, which contain four volumes of carbonic acid in every 10,000 volumes of air. Some small corrections are necessary for the accelerative effect of the force of gravity (g) at the latitude of the place where the air is weighed (the latitude of 45°, being referred to as a normal latitude for such weighings),

those for the 5 metres above

and also for the height of the place above sea level. The above weights of the cubic foot are latitude of Westminster, 51° 29' 53", at mean sea level (as determined by the mean level at Liverpool). In such latitude a litre of dry air would also weigh 1.293934 grams (t = 0°C., B = 760 millimetres), but a litre of such air freed from carbonic acid would weigh 1.293519 grams.