One of the most important points in connection with the filling of the blast-furnace is to maintain the stock line at the proper height. With hand-filling this was frequently very difficult to do. The physical limitations of the apparatus, to say nothing of those of the men, were such that if the furnace once "got down," that is if the stock line settled to a lower level than it should by the melting of the stock column below, it was very difficult to catch up, as I have already explained. If the stock line settles to the level which the stock should reach after it has been in the furnace for say two hours, then obviously the stock charged while that condition prevails is deprived of two hours of the time of treatment it should receive. This is frequently more than enough to derange seriously the working of the furnace.

For this reason it is of great importance that the furnace shall not only receive in one twelve hours the complement of stock which it requires, but that this should be received at a uniform rate, and the stock line kept constantly within a very few feet of its proper position. When the furnace is filled with skips their capacity for handling materials is so vast that there is not often any physical obstacle to prevent the furnace from being kept full, but the very speed with which it may be charged may be a disadvantage, since the men on the night shift may take a "spell" of an hour or two, knowing that by hard work they can catch up by daylight, although the furnace may be "down" many feet and its working deranged thereby, when they begin to make up for the time lost.

Gauging rods working through the top of the furnace, generally through the hopper or a point just back of its upper edge, have been provided ever since the introduction of the closed top, and in recent years, since the introduction of skip-filling, arrangements have been made whereby the test rod is suspended from a rope which winds on a drum at the bottom of the furnace, and by operating this drum the furnace can be gauged without the necessity of going on top.

There is also another need to gauge the stock line. This results from the fact that while the stock line ordinarily settles quite regularly, so many inches per minute, at times the furnace slips and the stock line drops from two or three feet up to fifteen or twenty in moderate slips, and of course much greater depths in violent explosions. Even though the furnace be working normally below, it is highly desirable to have a record of the way the stock line is settling, so that any tendency to lag or work irregularly can be detected and cured without delay.

For the accomplishment of this purpose I designed several years ago an apparatus called a "Stock Line Recorder," of which the principle is very simple. (See Transact. Amer. Inst. Min. Eng., Vol. XXXVI.) The test rod instead of being raised up out of action except when the furnace is to be gauged, rests normally on the stock and settles with it. When the bell opens to admit the charge the test rod is drawn up out of the way, and when the bell closes it settles back on the stock in its new position, and follows it down until lifted for the next charge, and so on.

The mechanism by which this is accomplished is extremely simple, the actuating portion of it is shown by Figs. 57, 58, 59. The test rod, which is made quite heavy, hangs in the furnace in the ordinary way. A light, flexible wire rope is passed over a sheave on top of the furnace, and from thence to any convenient point, preferably near the skip operator's platform, where the actuating mechanism is placed.

The rope is wound on the narrow flanged drum, shown at the top in the plan view. This drum is keyed to a shaft, on the other end of which is keyed a small pinion meshing with a rack fastened to the piston rod of the cylinder shown below. The piston rod works through a metal gland with a fairly free fit so as to avoid friction. The piston is made quite heavy or else weight is added to it in some way so that this weight acting through the pinion and drum tends to take up the slack from the test-rod rope, but on account of the much greater radius of action of the rope, and the considerable weight of the test rod, the weight of the piston alone cannot lift it. The top of the cylinder is connected to the main bell cylinder, generally without any valves or connections of any kind, so that when steam is turned on the main bell cylinder to open the bell, it is simultaneously turned into the top of the recorder cylinder, and forces the piston down, raising the test rod to its highest or "safe" position.

Figs. 57-58. Operating mechanism for stock-line recorder.

When the steam is exhausted from the bell cylinder to close the bell, it is simultaneously exhausted from the recorder cylinder, and the weight of the test rod with its greater leverage then lifts the piston in the cylinder until the test rod rests upon the stock, when it comes to rest, and only moves thereafter as the test rod settles in following the stock down. The movement of the mechanism is reduced by the lever shown in the back view (Fig. 58) and transmitted to the mechanical recorder, either of the dial or the strip type, and makes thereon a saw-toothed record as shown in Fig. 60.