Ans. The method to be employed in putting into operation a bench of retorts heated by a generator furnace will depend somewhat upon whether the bench has just been built or reset, or is one that has been previously fired up and let down.

In the case of a new bench, or of one that has just been reset, it is necessary that all the brick work about the bench should be thoroughly dried, by being subjected to a low heat for a long time, before any attempt is made to heat it up to a working point. This drying cannot be hurried without injury. to the bench, and at least two weeks should be allowed for it. It is effected by keeping a very low fire on the grate of the furnace, using about a bushel or two of coke at the start and keeping the fire about the same size afterwards. The clinkering door of the furnace should be left wide open, the object being to pass through the bench as much moderately warm air as possible so as to pick up and carry off the moisture. The primary and secondary air ports should be closed, and also the main dampers. When the fire is first made all the stoppers in the front wall of the setting should be removed, leaving the stopper holes open. An opening the size of one or two bricks headerwise should also have been left in this wall directly under the crown of the arch. As the main dampers are closed the warm air laden with the moisture picked up in the furnace and lower part of the setting escapes through these openings in the front wall, carrying the moisture with it, while if it had to pass all around the retorts and through the take-off flues it would become cool and deposit most of the moisture in these flues before it escaped to the chimney. After the furnace is dried the stoppers can be put in the lower openings and the warm air forced to travel farther along, and so on, until after a week or ten days the main dampers can be opened a little, the large opening under the crown of the arch closed up and the warm and now comparatively dry air drawn through the whole of the setting and the take-off flues, thus drying all portions of the bench.

After the drying out is completed the bench is brought up to working heat in exactly the same manner as a bench that has been let down after a certain amount of service, except that a new bench should be heated up a little more slowly than an old one.

For heating up, a slightly larger fire is built than is used for drying out. Either just before or immediately after this fire is started, any coke left in the retorts from previous working must be drawn, the retort mouthpieces closed, the secondary air ports closed, and all stopper openings and cracks in the brick work carefully plastered up so that no air can get into the bench except through the primary air flues and the clinkering door. The primary air ports are opened slightly, as are also the main dampers. The clinkering door, instead of being left wide open, as in drying out, is only left slightly ajar, since now the object is to have the fire heat the bench and not to have it heat an excess of air passing through the bench. It is necessary to leave the door open a little to admit air above the surface of the fire for the purpose of consuming any carbonic oxide (CO) that may be formed as soon as it escapes from the fuel, and thus prevent the formation of an explosive mixture of carbonic oxide and air in the upper part of the furnace and in the setting, since such a mixture, if allowed to form, would probably be ignited later and explode with more or less damage to the bench. It is also necessary that the surface of the fire should never be entirely covered with cold coke, some portion being always left hot and incandescent to ignite the carbonic oxide as soon as it meets the air.

The fire is gradually increased until at the end of from 48 to 96 hours it can no longer be fed through the clinkering door. The plates and water pan of the step grate are now put in, the clinkering door shut, the water started running on the grate, and the primary air ports and main dampers opened wider. The fire is fed through the filling door until the furnace is half full. If cold coke is used the precaution noted above of never entirely covering the hot surface of the fire must be carefully observed. If hot coke is used, this, of course, obviates any danger of a cold surface to the fire. As soon as the combustion chamber and lower retorts reach a good red heat and the carbonic oxide coming from the furnace is seen to burn at the nostrils when air is admitted to the combustion chamber, the secondary air can be turned on and the bench brought up to heat as rapidly as is necessary, the furnace still

being kept about half full of coke, being completely filled only a few hours before the bench is to be put in action.

As soon as the retorts reach a red heat they should be patched and charged with coal, the charge being left in until burnt out. The bridge pipe covers should, of course, be closed before the retorts are charged. When the bench is nearly up to its proper working heat, regular charging can begin, and when this heat is reached, the openings of dampers and primary and secondary air ports must be carefully regulated to the proper respective areas.

If necessary, an old bench can be put in action within sixty hours after the fire is first started, but it is best to proceed more slowly, if possible, and spend five or six days in bringing the heat to the working point. (Trustees.)

Describe the cleaning of the washbox of a carburetted. water gas apparatus, including the precautions to be observed to prevent the occurrence of any accident.

Ans. There are two arrangements of washbox in water gas apparatus. In one the take-off from the washbox is on top, and in the other it is on the side, and connects directly with the scrubber.

The connection from the gas outlet on top of the superheater to the washbox varies in different forms of water gas apparatus. In most cases there is a lid on top of what is known as the oil heater connection, which can be opened to clean the oil heater.

When no gas heater is used, the take-off connection from the superheater has a handhole cross at the top of the superheater, connecting the vertical riser from the washbox to the outlet branch on the superheater.

When the washbox has a take-off on top, there is a valve between the washbox and the scrubber, which can be closed and thus shut off communication between the washbox and the scrubber. In this case, first open either the lid on top of the oil heater, or, in case there is no oil heater, the handhole on the cross; then shut off the overflow from the washbox to the seal pot, open the handhole on top of the washbox and fill the washbox with water. When the washbox has been filled,

draw off the water, open the handhole or manhole on side of washbox and remove the tar, etc.

In case the washbox is joined to the scrubber by a side outlet, and there is no valve between them, the gas valve at the outlet of the scrubber must be closed. The stop cock on the overflow pipe from the scrubber must now be shut and the manholes on the scrubber opened, beginning with the one on top and working downward. The openings on the take-off pipe from the superheater to the washbox and the handhole on top of the latter must also be opened up, and the washbox filled with water until this rises into the standpipe above the top of the box. This, of course, involves filling the scrubber also to an equal height. Before emptying the washbox, the standpipe should be thoroughly cleaned of all carbon and tar to remove all danger of its catching fire and igniting any gas. that might work back from the scrubber when the water is run out. If any of the carbon should happen to be on fire when it is scraped down, it will be extinguished by falling into the water, and before the water is drawn off, care must be taken to see that none remains unextinguished. After the standpipe has been cleaned, it should be plugged, just below the connection to the superheater, with either a bag of wet sawdust or a board cut to fit it fairly tight and plastered around the edge with fire clay to close all cracks. The object of this plugging is to prevent any gas that may work back from the scrubber from being ignited at the top of the superheater. The water can now be run off the washbox and scrubber, and the washbox opened and cleaned.

The differences between the two methods are called for by the necessity of seeing that the scrubber is thoroughly ventilated when it cannot be shut off, and of taking precautions to prevent the ignition of any gas that may still remain in the scrubber in spite of the ventilation given it. In any case, the washbox and its connections must be thoroughly ventilated, and it should be provided that no fire comes near the washbox or its connections, or the scrubber, while they are open, and that no light is used about the work. (Trustees.)

4. Give a description, illustrated by a sketch, of the

manner in which to stop a small ragged hole in a gas holder sheet too thin to permit of tapping a thread.

Ans. Make the hole oblong, say 1 in. to 1% in. long by in. wide. Flatten the head of a 4 in. bolt until it is only 4 in. wide. Make a cast iron or other washer, 2 in. in diameter, with the hole a tight fit over the bolt. Mix a stiff paste of glycerine and litharge, red lead or any similar substance. Tie a string to the bolt, put the flattened head through the hole, give it a quarter turn, slip the washer, coated with the paste, over the bolt, grummet this with lamp wick, screw down the

nut until everything is drawn up tight, saw off the projecting end of the bolt, and the job is complete. (Trustees.)

5. What is the meaning of the term "latent heat of a vapor," and what is the value of this "latent heat" in the case of steam?

Ans. "Latent heat of a vapor is the quantity of heat which must be communicated to a liquid in order to convert it into a vapor without change of temperature." (Maxwell.)

When heat is supplied in sufficient quantity to a substance its temperature is increased up to a certain point at which, provided