of mains should be reduced to the equivalent length of say 3′′ main before the division is made.

For comparisons between different periods in the history of the same company, the percentage system is only correct as long as the sales of gas per mile of main remain about the same. When the sales change decidedly, a decrease in the percentage may coincide with an increase in the amount of actual loss. If, for instance, the company with 60 miles of main increased its sales to 100,000,000 cubic feet per year while its main mileage was increased to 65 miles, a loss of 8% would amount to 8,000,000 cubic feet per year equal to 118,000 cubic feet per mile of main as against a loss o only 83,000 cubic feet when the percentage was 10.

There are of course, cases where conditions are so different that even the loss per mile of main per year does not afford a basis for accurate comparisons, but as a general rule, the conditions which affect the accuracy of such a comparison do not vary as much as do those which affect that of the comparison on the percentage basis. (Trustees.)

II. Describe some method of coating wrought iron pipe, such as is used for services, or some other way by which this pipe can be protected from corrosion.

Ans. The method of coating service pipes that seems to have given the best results, is that which was first employed in Cincinnati after a series of tests made with a number of different coatings. The specifications for this coating are given, and the manner in which it should be applied to the pipes to give the best results is described, in a "Short Topic" prepared by Mr. A. C. Humphreys for the 1895 meeting of the American Gas Light Association, and published in Vol. 13 of the Proceedings, page 186.

The specifications are as follows, "Bring a kettle of tar (20 gallons) to a low boiling point and add 20 pounds of fresh slaked. lime, sifted over the top and worked down. Boil down to a paste or a consistency about midway between tar and pitch. Let it settle for a few minutes, then add 4 pounds of tallow and 1 pound of powdered rosin; stir until they are thoroughly dissolved and incorporated with the tar, then let it cool and settle. Ladle off into barrels. When ready for use, to each barrel of 45 gallons of the above mixture add 4 pounds of crude rubber, dissolved in turpentine to the consistency of thick cream. Heat the mixture to about

100% Fahrenheit and immerse the service pipe, heated to about the same temperature."

Different methods have been adopted for applying this mixture, the pipes being dipped in some cases and painted in others. Mr. Humphreys said that the best method was to dip the lengths of pipe into the preparation. In this method the mixture is put into a V shaped trough which is long enough to take the longest length of pipe that is apt to be handled. Along the bottom of this trough is run a steam pipe to heat the mixture to the proper temperature. The pieces of pipe to be coated must also be heated, which can be done either by connecting the pieces to a steam pipe and turning steam on them to give them a test of soundness while they are being heated, or by putting them in a long square box with a hinged cover, the interior of the box being kept hot by means of a steam coil run along the bottom. When the pipes are heated to the proper temperature, they are plugged at both ends to prevent any of the coating from getting on the inside, and are dropped into the hot mixture in the trough. After a few minutes they are taken out, placed on a rack over the trough until the excess of the mixture has dropped off, and then transferred to a drying rack in which they are left until the coating has become thoroughly dry and hard. Care must be taken to keep the pieces from touching each other before they are dry, since, if this is not done, they will stick together and when pulled apart, the coating will stick to one pipe and a bare place will be left on the other. Care must also be taken not to get either the mixture or the pipe too hot, since overheating will drive off the more volatile portions of the preparation, and when the coating cools, it will be brittle and liable to chip off. If the proper precautions are taken and fresh portions of the mixture added from time to time to that already in the trough, the coating will be elastic and will cling very tightly to the pipe. The service gang should, however, be supplied with a pot of the preparation so that after the service is laid the spots where the coating has been scraped off by the tools can be re-coated before filling in the ditch. In some cases the pipe, as it lies in the ditch, is covered for its whole length with an extra layer of the coating, which forms a roof over it.

It will be found rather difficult to dissolve the rubber, but this part of the specifications must not be slighted. Given proper care in the preparation and application of this coating, the pipe cov

ered with it will last much longer than uncoated pipe, and the cost of coating will be more than repaid by the extra life of the services and the lower leakage account. (Trustees.)

12. What is the so-called "Otto cycle " in gas engines?

Ans. The so-called "Otto cycle" in gas engines is a four phase cycle carried out in a single acting cylinder open at the end toward the crank, and provided with a tightly fitting piston which is free to move back and forth.

Starting with the piston at the beginning of an out-stroke the cycle is as follows: As the piston moves out on the forward stroke, the charge of gas and air is drawn in and fills the cylinder. The in or back stroke compresses the charge into the clearance space. At the dead point at the end of this second stroke, the charge is ignited and the resulting explosion drives the piston out on its second forward stroke. The succeeding in-stroke expels the products of combustion from the cylinder and the cycle is completed, the piston being in its first position and ready to repeat the same series of operations. It will be seen that the cycle requires four strokes of the piston for its completion, and that of these four, the third is the only motor stroke, the power required to carry on the others being derived from the energy stored in the fly-wheel during the third stroke. It will also be seen that there is only one explosion for each two revolutions of the fly-wheel.

This cycle was first described and advocated in 1862, by M. Beau de Rochas, and is properly called the Beau de Rochas cycle, but it was not put into practical use until 1876 when it was adopted in the Otto Silent Engine, the first gas engine that was commercially successful and one which held the field almost to the exclusion of all others until the patent expired. (Trustees.)

ANSWERS TO SECOND SERIES OF QUESTIONS, 1901-THIRD SECTION-PRACTICAL CLASS--AMERICAN GAS LIGHT

ASSOCIATION.

I. Name and describe the three grades of oil which are commonly used as the carburetting material in the manufacture of carburetted water gas.

Ans. The three grades of oil which are commonly used for car

buretting water gas are crude oil, naphtha and gas oil, the last two being obtained from the first by fractional distillation.

Crude Oil is petroleum just as it comes from the oil wells. It is a mixture of liquid hydrocarbons possessing different chemical compositions, specific gravities and boiling points (together with small quantities of other substances) and can be separated into these different substances by fractional distillation. It is found in nearly every country in the world, but the most important fields are in the United States, Russia and Peru. The specific gravity of the petroleum obtained in Pennsylvania and Ohio varies from .80 to .85 water being 1.00. The color also varies but is generally dark claret by transmitted light and greenish by reflected light.

Naphtha is a general name given to that portion of the products of the fractional distillation of petroleum which comes off after gasoline has been driven off and before the fraction suitable for use as burning or lamp oil begins to come off. It is very volatile at ordinary temperatures and highly inflammable. It is practically colorless though some samples show a decidedly greenish tinge by reflected light. Its specific gravity varies from .68 to .74.

Gas Oil is a general name given to that portion of the products of the fractional distillation of petroleum which comes off after the lamp oils and before the lubricating oils. It is too heavy and of too high a boiling point to use for lamp oil, but not viscous enough to make a good lubricant, and is chiefly valuable for the production of gas. Its color varies from quite light to very dark. Its specific gravity also varies but will average about .84. (Trustees.)

2. Describe the operation of a direct fire furnace used for heating coal gas retorts, paying especial attention to the way in which the fire should be handled to secure the most economical results.

Ans. When a direct fire furnace is employed for heating coal gas retorts the carbon of the fuel is burned to carbonic acid (CO2) in one operation, which is practically completed in the fuel bed, and the retorts are heated principally by the sensible heat possessed by the products of combustion passing off from the fuel bed. It is therefore necessary to so handle the furnace as to produce the highest possible temperature in the fire itself. In fact the fire should be handled very much the same as a boiler fire. The bed of fuel should not be made too deep, a depth of from 15" to 18" giving the best results. Fresh fuel should be added at fre

quent intervals, about once every hour, in order to maintain this depth constant and keep the fire from burning down too low. The grate bars should be shaken lightly from time to time so as to keep the bottom of the fire bright. The condition of this part of the fire can be determined by observing the reflection shown on the water in the ash pan. About 12 hours after the fire has been clinkered a slice bar should be run along the grate bars and the clinker that has formed there should be thoroughly broken up. Clinkering must be thoroughly done every 24 hours. It is impossible to use secondary grate bars when cleaning a furnace of this kind, and so after pushing as much of the fuel as possible up on the slope of the back wall the rest is dropped into the ash pan by drawing away the grate bars, the clinker is cut away from the side and back walls and removed from the furnace, the grate bars replaced, the fuel on the back wall drawn down and spread over the bars and fresh fuel charged into the furnace. The wear and tear on the furnace walls can be reduced by leaving an inch or so of clinker on them at all times, never attempting to take it off down to the brick.

To secure economy of fuel it is necessary to prevent any air from being drawn into the setting through cracks in the brickwork and openings around the frame of the charging door, and to be careful not to have an excessive draft. A direct fire furnace requires more draft to maintain the heat in the bench than does a generator furnace, but an excessive draft is bad in both cases. The appearance of flame at the top of the chimney flues indicates the existence of too much draft, and when it is noticed the dampers should be pushed in.

All flues should be carefully cleaned out at least once a month in order to prevent any stoppages in them that would interfere with the keeping up of the heat, such stoppages being very frequently the cause of low heats when everything is apparently in good order. (Trustees.)

3. Give a description, illustrated with sketches, of one length of hydraulic main for benches of six retorts, including the dip pipes, overflow or overflows, arrangements for cleaning out heavy. tar or pitch, and the considerations that determine the area of the surface of liquor in the main outside of the dip pipes.

Ans. The hydraulic main for a bench of coal gas retorts should be made of wrought iron. The cross section shown on the accom