This is controlled by various circumstances. When the ore supply for the large part of a year has to be stocked that becomes one of the controlling factors because the cost of ore bridges increases very rapidly with their length, and commercially their practical length is limited on this account, also by the great time required to traverse the trolley from one end to the other of a long bridge. The height of the ore pile is limited by considerations affecting the cost and strength of retaining walls at the sides of the stock pile, and the slope which the ore takes running back from them. The pile is thus limited in two dimensions, and in order to store a given tonnage of ore the required volume must be obtained by making the length of the pile per furnace as great as is necessary, and this length is obviously the minimum distance between furnaces, for those conditions.

There are many other considerations affecting the spacing of the furnaces, however, especially in those cases where considerations of the size of the stock pile do not control them. It is very undesirable to have the furnaces crowded together as it is more difficult to work around them under those conditions, more damage is likely to be done to one furnace by an accident at another, and questions of accessibility of various kinds arise which it is practically impossible to pro-vide for in detail in advance, and which can only be met satisfactorily by having a certain amount of room around each furnace.

On the other hand, it is undesirable to have them spaced too far apart on account of the initial expense of land, tracks, etc., and such spacing not infrequently defeats its own ends by offering a temptation to put something into the unoccupied space, another furnace or some other part of the plant. This has happened on more than one occasion. The supervision of furnaces is also more difficult when they are scattered over too wide a territory. One blower can frequently blow two furnaces if they are spaced close together, but would not be close enough to the work to manage two located very far apart. When the stoves for each of a pair of furnaces are arranged in a continuous row as shown in Fig. 236 this in itself controls the minimum distance apart of the furnaces, as it virtually does at that plant.

Furnaces are very generally arranged in pairs, rights and lefts, as shown in Fig. 236, and more clearly shown in Fig. 238, which illustrates the arrangement of a pair of furnaces with their stoves, gas mains, etc., in some detail. On the other hand, at one of the best built and most successful plants the plan was adopted of having the furnaces all built the same "hand," that is to say, looking at the plant from the front, the stoves are all on the left-hand side of their respective furnaces.

Fig. 238. General ground plan and elevation of a pair of furnaces with their stoves and dustcatcher. Indiana Steel Company, Gary, Ind.

One of the advantages of the pair arrangement is that it makes the distance between furnaces alternately very short and quite long. The close grouping of one pair of furnaces is an advantage since that is about the limit of what a single blower can supervise satisfactorily, and having these two close together makes it possible for him to keep in much closer touch with their operation. The fact that the next pair is several hundred feet away is of no importance whatever to him.