outer cylinder, A, and its top fits snugly against the vent opening at the top of the cylinder. The axis of each of the

tubes and that of the cylinder all coincide, and the annular space between the tubes is about 1" wide.

Gas is supplied by means of a pipe, E, which should be 34" in diameter. A pilot light is used to light the gas at the main burners as shown. The lever of the main gas cock has on it an arm so arranged that as the gas cock is opened this arm opens the main water cock on the pipe, F. It is thus impossible to turn on the gas without turning on the water, and there is no danger of the heater being damaged by being exposed to excessive heat owing to the gas being turned on while the water is shut off. After passing the main cock the water goes through a regulating valve, G, by which the quantity flowing can be regulated to suit the requirements as to the temperature at which it is to be delivered. It is then carried, by a pipe inside the inner tube, up to the top of this tube. From the end of this main supply pipe branches are led to deliver the water directly on the inner surface of the inner tube and the outer surface of the outer one. One of these branches is shown at H. The water flowing over the inner tube follows the corrugations to the bottom, whence it flows through the pipe, J, into the bottom of the space between the outer tube and the cylinder, in which is also collected the water that has followed the corrugations of the outer tube. From this space it is delivered through one or more pipes, K, to the point, or points, where it is wanted.

As will be seen from the cut, the only outlet for the products of the combustion of the gas is up through the annular space between the corrugated tubes. This space being narrow, the products are brought into close contact with the metal and rapidly impart to it their heat to be taken up by the water flowing over it. As the products and the water travel in opposite directions, the water as it is heated flows over surfaces that are more and more highly heated and so is able to take up heat until it leaves the heater. The products of combustion, becoming cooler and cooler as they ascend, finally leave by the vent opening, which should always be connected to the outside air.

A

gauge glass, L, is provided to show the height at which the water stands in the space between the outer tube and the cylinder, and thus call attention to the necessity for checking

the flow by the regulating valve should the water be running in faster than it can be carried off by the delivery pipes.

Heaters of the style shown on the cut contain 10 to II square feet of heating surface, will raise the temperature of the water by 50° F. when it is flowing at the rate of from 11⁄2 to 2 gallons per minute, and will consume about 1 cubic foot of gas for each 1 to 11⁄2 gallons of water heated.

As will be seen by the cut, the water and the products of combustion do not come into contact with one another in such heaters, and the water heated by them can be used for cooking as well as for washing. Another style of heater is made in which the water and products of combustion do come into contact with each other, and such heaters can be used only for water that is not to be taken internally. (Trustees.)

12. State some of the reasons for the increasing use of Portland cement concrete in the place of brick or stone

masonry.

Ans. One of the principal reasons for the increased use of Portland cement concrete in the place of brick and stone masonry is found in the growth of knowledge as to the advantages and capabilities of concrete. When necessary, it can be made to excel in strength and durability all other masonry except that made from the very best stone, while when great strength is not required it can be made less strong, with a reduction in cost that cannot be had under similar circumstances with brick or stone masonry. On foundation work it can be put in with a smaller expense for excavation and will give a more coherent, homogeneous structure, and one that is less subject to unequal settling, in case the soil is not uniform over its whole area, than will be the case with stone. It can be made water tight more easily than brick or stone. Under ordinary conditions as to the respective costs of the different kinds of materials it is cheaper than brick or stone masonry.

In addition to the recognition of its possession of the advantages named above, another important reason for the increased use of concrete in large cities is that it can be made and put in place by unskilled labor, and work in which it is

used instead of brick or stone is thus free from the vexatious and costly delays that are apt to be experienced with other masonry, owing to the strikes declared for trivial causes by the masons' unions to which practically all the masons in such cities belong. This ability to use unskilled labor also frequently permits of the work being executed more rapidly on account of there being available more laborers than brick or stone masons. These considerations of freedom from union interference and ability to secure unskilled labor more easily than skilled, have been very important ones in large cities during the past few years. (Trustees.)

I.

TWELFTH SERIES OF QUESTIONS SECTIONS OF 1906PRACTICAL CLASS-AMERICAN GAS LIGHT

ASSOCIATION.

A net ton of one of two gas coals that are available for use yields 9,200 cu. ft. of gas, 750 lbs. of coke for sale, 10.5 gallons of tar and 4.5 lbs. of pure ammonia, while a net ton of the other yields 9,700 cu. ft. of gas, 850 lbs. of coke for sale, 15 gallons of tar and 5 lbs. of pure ammonia. The average make per retort per 24 hours is 8,300 cu. ft. with the first coal and 8,900 cu. ft. with the second. In both cases retort house labor costs $4.00 per each six retorts per 24 hours and the prices for the products are: Gas, $1.25 net per 1,000 cu. ft.; coke, $4.00 per net ton; tar 3c. per gallon, and ammonia, 5c. per pound. The first coal costs $2.40 per net ton. What is the equivalent price of the second?

2. Describe briefly and generally some method of charging and drawing gas retorts, in which hand labor is assisted by mechanical appliances, so arranged as to render the work easier and enable it to be done by fewer men.

3. Give a description, illustrated with sketches, of some form of hand pump used for pumping the drips on the inlet. and outlet pipes of gas holders in brick tanks.

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12.

In the manufacture of carburetted water gas it has been found that with the ordinary methods of condensing and tar extracting some vapors remain in the gas until the purifiers are reached and are then deposited in the purifying material with the effect of shortening its life. What has been found to be an efficient way of removing these vapors before the purifiers are reached?

What is meant by electrolysis as applied to gas mains and what is its chief cause?

What are the respective advantages and disadvantages of cast and wrought iron pipes for use as gas mains ?

Give a description, illustrated with sketches, of some form of recording pressure gauge used for taking street main pressures.

How should persons overcome by gas be treated?

In putting in gas piping that will be exposed to extreme cold, such as the risers in street lamp posts or the portions of mains on bridges, what method would you adopt to guard against the obstruction of the pipe by frost ?

What is the “Principle of the Conservation of Energy," and how would you apply it to check up the claims made as to the results to be obtained from a new process for making gas?

Describe the cycle or series of operations and the method of governing employed in each of the following gas engines Otto, Westinghouse and Körting.

Should the lining in the generator of the carburetted water gas apparatus be made single or double? Give the reasons for your answer.

(Answers to these questions are due February 1st, 1906.)