We fancy that few of our readers concern themselves with radiant heat. Men seem to hold that it is of interest for the physicist only-not for the engineer. No doubt it presents curious facts for consideration, but it is thought that they have few or no practical applications. Thus, if we follow a paper on steam boilers, or listen to the discussion which ensues, we find that attention is concentrated on the conductivity of plates, the absorbing powers of water, the emissive capacities of hot gases in direct contact with tubes, and so on; but nothing, or next to nothing, is said about radiant heat. Why this should be the fact is not easy to say. The neglect of radiant heat goes far to explain certain mistakes made in boiler engineering, and it is desirable that the whole subject should be studied and dealt with, just as other forms of heat have been considered in all ways, and deductions drawn for the benefit of the steam user.

In "Heat as a Mode of Motion" Tyndall said what until quite recently was regarded as the last word about radiant heat; and to his book we must refer those of our readers who seek information in detail. Our concern at present is not with the pure science of the matter, but with the part played in the everyday generation of steam by radiant heat. Two boilers will exist apparently not very dissimilar-one will be an economical steam maker, and the other will not. Again, the same boiler fired in different ways will give different results. Apparently minute variations in dimensions produce results whose proportions are startling. Of course many causes may contribute to produce this diversity. But none is likely to be more influential than the way in which the radiant heat of the burning fuel is dealt with. In theory the effect of radiant heat varies inversely as the square of the distance of the hot body from the cold. But this bare statement by no means covers the whole of the facts. Let us suppose that an incandescent cube of one foot is placed inside a box with a capacity of a cubic yard, and surrounded with water on every side.

Water will then be evaporated at any given rate. Now let the incandescent cube be placed inside a box double the size of the first, surrounded in the same way with water. Theoretically just as much water should be evaporated in the second case as in the first.

cube is, it is true, greater; but the heating surface is larger, and in any case the incandescent mass has a fixed quantity of thermal energy to dispose of, and it must apparently go to the water. If not, what becomes of it?

If we apply this theory in practice we may have as large a firebox as we please, and the distance of sides or crown from the burning fuel will not affect in any way either the potential or the economic efficiency of the boiler. The heat is there. It can not get away. It strikes in right lines. It fills the whole box. When we try the experiment, however, nothing can be further from the truth than this reasoning. To get hold of radiant heat the plates must be near the fire. The big firebox has been tried over and over again, and it has always failed. No one knows what becomes of the radiant heat. In apparently a mysterious way it seems to be lost. It is beyond question that some boilers give unexpectedly good results because they catch the radiant heat. Others fail simply because the heating surface is too far away from the radiant fuel.

The application of this truth is of considerable importance. It is highly desirable that the space above burning fuel should be lofty, so that plenty of room may be present for the admixture of air with the hot gas, that smoke and waste shall be prevented. But the big combustion chamber means removing the wet plates far away from the incandescent fuel. The boiler will not steam well. The late Sir W. Anderson was one of the few men who said that if theory is not consistent with the common everyday facts of life so much the worse for theory. It is notorious that boilers which have made plenty of steam while their furnaces gave off smoke, did so no longer when smoke was prevented. An interesting illustration of this occurred some years ago, when Thames tug boats were fined for making smoke. It was pointed out that when smoke was prevented they could not do their work. Sir William Anderson showed that this was in the main due to the fact that the clear blue flame of perfect smokeless combustion gave out little radiant heat; while a lurid, smoky flame is a powerful radiator, and with it the efficiency of the flue heating surface was at its best. If a Bunsen flame is made to traverse without touching a vertical tube surrounded by water, it will be found that scarcely

the trouble is caused by defects of a more serious nature and more difficult to emedy. The clinkers on a previous trip may have run together and stopped up the openings in the grates, or the fire may be much too heavy for the way the engine is drafted or is being worked, or too much air may be entering the firebox above the fire, due to a poor joint at the fire door, or too many hollow staybolts, or the combustion tubes or the vacuum in the front end may be insufficient as a result of one or more of the following causes: Front end netting may be stopped up; a steam pipe may leak, which will permit of escaping steam supplying the vacuum instead of air, which should come through the fire to supply it; or the joint at the bottom of exhaust stand may leak steam. Leaky flues in the front end or in the firebox, or, in fact, any steam leak in the place where the vacuum should be maintained, will have the same effect.

The vacuum depends for its existence on the jet formed by the exhaust passing properly into the stack. Should the exhaust stand happen to be out of line the jet will strike the bottom edge of the stack and spread, thereby almost, if not entirely, destroying the vacuum, and, consequently, the draft. It often happens that when an engine slips under such conditions the gases are driven back through the flues and out through the fire door. In case the exhaust nozzle is too low the effect will be the same, as the exhaust jet spreads before it strikes the bottom of the stack, and much of the steam is thus retained in the front end.

If the exhaust nozzle should be too high the exhaust jet will escape without properly filling the stack, in which case the suction necessary to create a vacuum is incomplete, and it is therefore only partially formed and not so effective. Should a petticoat pipe happen to be out of line with the nozzle and stack it would have the same effect as if the nozzle itself were out of line.

It sometimes happens that the diaphragm is placed too close to the flue sheet, thereby restricting the flow of gases and so reducing the draft, and, again, the nozzle may be too large for the engine or for the class of coal being burned. When an engine tears her fire all over it indicates too sharp a draft, the remedy for which is to enlarge the nozzle. The fire burning too fast next to the flue sheet is an indication that the draft through the lower flues is too strong, which may be due to the dia

phragm (or deflector plate) being too low, the upper flues being honeycombed over, or the fire being banked or clinkered at the back end. This is often remedied by closing the front damper. If the engine pulls her fire from the back end of the firebox it shows that the draft is too strong through the upper flues, which may be due to the draft sheet being too high or the lower flues being stopped up.

It happens occasionally that the draft is so intense as to pull the coal forward and pile it up against the flue sheet. In order to remedy this it must be arranged to get a stronger draft through the lower flues and to keep them open, which can be done by following the information already given in this article. If the engine burns her fire evenly and seems to have good draft, but still does not steam, it may be that the fire is not getting a sufficient supply of air, or it may be that it is getting too much air. It is difficult sometimes to tell, particularly with engines having wide fireboxes, whether a fire is getting too much or too little air. It can generally be determined by making the following observations: If when the fire is light no puff of smoke appears at the stack after putting in a fire, it indicates that the fire is probably too light and should be built up a little, but gradually. If the fire looks dark and smoky, but immediately clears off when the door is opened, it indicates that the fire is too heavy and should be burned down at once.

It requires about twice as much air as will fill an average box car to burn one scoopful of coal. In order to have steam at all times the engine must be not only fired properly, but handled properly as well. Although the draft, air supply, firing and handling of an engine may be perfect, the engine will not steam satisfactorily if the boiler contains large quantites of mud and scale, or if many of the flues are plugged or stopped up so that the heat of the fire can not be properly utilized by universal application to the heating surface. When the number of flues are materially reduced by plugging or stopping up, the gases pass through the remaining flues and out the stack much hotter than they would if distributed over the entire flue surface, and great heating effect is thus wasted.

While many boilers make sufficient steam for the use of the locomotive, they are unable to furnish the additional amount necessary to supply leaks occasioned by balance strips, valve seats, packing, etc. Heat derived from different