back of the case as determined by sighting from one line to the other. In the old form of these photometers the scale on the gauge is graduated to hundredths of an inch and the candle power, corresponding to the pressure which exists when the flame is adjusted to the proper height, is obtained from a table furnished with the instrument, either separately or in the form of an auxiliary scale placed below the pressure scale and properly tied into it. In the more modern forms there is no pressure scale as such, the semicircular scale over which the pointer of the gauge travels, being graduated to give the candle power by direct reading, as shown on the accompanying cut. It is of course important that the gauge should show the correct pressure, and to do this the water line must be maintained at the proper level at all times. The adjustment of the water line is provided for by a small cylindrical tank mounted on the side of the case opposite to that on which the gas cock is placed. This tank, which communicates with the gauge by a connecting pipe, is provided with a piston, and the space below this piston being filled with water, the water line of the gauge can be readily adjusted to the proper level by moving the piston up or down according as the water is above or below the proper point. Another style of jet photometer, of which the Jones Jet is an example, determines the candle power of the gas, from the varying height of a flame produced by burning the gas through a small orifice at a constant rate or at a constant pressure. The Jones Jet consists of a steatite tip with a fine circular orifice to which the gas is supplied by a volumetric governor and the candle power scale is marked on a colored glass chimney surrounding the flame. If illuminating gas could be made at all times of a uniform composition, a jet photometer if once correctly adjusted would give fairly accurate absolute candle-power results. Since it is not possible to make such gas of a perfectly uniform composition such a photometer cannot be depended upon to give absolute results that are accurate, as even when the tip is of the proper size for the kind of gas to be tested and the readings are correct when the gas has a certain composition, any change in this composition will change. the readings even though the illuminating value remains the same. It is obviously entirely out of the question to attempt, as is sometimes done, to use the same jet photometer without change of tip for both coal gas and carburetted water gas, or a mixture of coal gas and carburetted water gas. The true field of the jet photometer is to furnish a rapid method of obtaining frequent tests of candle power that can be compared with one another and thus afford a check upon manufacturing results that can not be obtained by comparatively infrequent tests made upon a bar photometer. But even when used in this way the reading of a jet photometer should be compared every day or two with the candle power as shown by a bar photometer in order to be certain that through change in composition of the gas or in the size of the orifice in the tip, these readings do not vary too much from the actual candle power of the gas, (Trustees.) 8. What do you consider the smallest size of cast-iron pipe which it is advisable to lay for street mains, and what weight per foot or per length of 12 feet do you consider suitable for this size of pipe? Ans. It is not good practice to lay any cast-iron pipe of a smaller diameter than 4", except in localities where it is reasonably certain that a smaller pipe will be able to deliver for an indefinite period, without undue loss of pressure, all the gas required in the territory to be supplied. A good average weight for 4" inch cast-iron pipe is 220 lbs. per length of 12 feet, or about 18 lbs. per foot. For use in very small towns lighter pipe can be employed, while in some locations in large cities heavier pipe is needed, but for general use the above is as stated good average weight. (Trustees.) 9. Describe, with sketches, a cross, or four-way branch, for four-inch cast-iron pipe, giving the dimensions to be used when it is provided with bells, and also those to be used for flanged ends, including the number of bolts and diameter of bolt circle. Ans. A 4 cross or four-way branch made according to the American Gas Light Association standard is shown on the accompanying cut. The bell cross has a bell on each outlet unless otherwise specified, although any desired combination of bell and spigot ends can be made. The distance from the center line of each run to the bottom of the bells on the other run is 6". The depth of the bells is 4" and the other dimensions are as shown on the cut. For a flanged cross the distance from the center line of the runs to the faces of the flanges is 6" in each case. The outside diame ter of the flanges is 9", the diameter of the bolt circle is 7" and the flanges are drilled with four 3/4" holes placed as shown. (Trustees.) IO. What details of construction have been adopted in the manufacture of flat flame burners in order to ensure that the combustion of the gas shall take place as far as possible under the conditions stated in the answer to Question No. 10 of the Second Series for 1901? Ans. The following details of construction have been adopted to put into effect the principles brought out in the answer to Question No. 10 of the Second Series for 1901. To ensure the existence of a low pressure at the burner tip the improved forms of flat flame burners are provided either with some form of governor, which maintains the pressure at the tip constantly at the proper point, no matter how much the pressure on the piping increases, or else with a "check," which is usually a metal, steatite or lava disk inserted in the burner pillar so as to cut off any flow of gas to the tip except through a hole in the disk, the area of which is smaller than that of the opening in the tip, the relation between the area of the opening in the check and that of the opening in the tip varying with the pressure at which the burner is designed to be used; that is, the higher the pressure the smaller the hole in the check for the same sized tip. In order to maintain the temperature of the flame at as high a point as possible the burner tip should be made of a non-conducting substance, since if made of metal or any other good conductor of heat the heat which is absorbed and carried away by the tip reduces the temperature of the flame. In all of the best forms of flat flame burners the tips are now made of either lava, steatite or enamel, all of which are poor conductors of heat. To produce a steady even flow of gas without any swirling motion some burners have placed between the check and the tip a screen of fine wire gauze which breaks up any currents, and renders the flow of gas uniform throughout the whole area of the burner pillar, while others depend upon the steadying action produced by the large area of the burner pillar above the check as compared with the area of the opening in the tip. To secure an equal supply of gas to all parts of the flame, slit (batswing) burners are made with what is called a hollow top, by |