For a long time after the introduction of the bell and hopper to close the top of the furnace no effort was made to prevent the escape of the gas during the time that the bell was opened for the introduction of the stock, but with the increased appreciation of the value of this excellent fuel it became more desirable to prevent even this waste, amounting to some 3 to 10 per cent. of the total gas produced, and for this purpose a secondary valve or seal was sometimes provided which was closed before the main bell was opened, and opened only after the main bell was closed.

One of the earliest forms of this apparatus consisted of a huge iron lid like a pot lid, which was lowered down on to the top of the hopper before the bell was opened, and raised again after the bell was closed.

The introduction of mechanical filling made the closing of the furnace top while dumping the main bell, easier and more desirable, and the obvious method of doing this was by the use of an apparatus similiar to the air lock used in caisson work which in this case becomes a gas lock in which one valve opens from the outside into the lock, the other opens from the lock into the furnace. The bell having so firmly established itself as the only valve worthy of consideration for closing the furnace top, it was perfectly natural that a similar bell should be used for the top valve of the gas lock, and this has become almost universal practice. This was the plan adopted at the Edgar Thompson Works, and in several other designs.

The development of the two-bell type of furnace top made the closed position of the gas seal bell the normal one, and this is greatly to be preferred because with mechanical filling it is possible to operate for a considerable period with the main bell leaking quite freely, if the gas seal be reasonably tight, and if it is closed in its normal position. But if the normal position of the gas seal bell be open, the leaking gas meeting the air burns up through the throat of the furnace and is likely to do great damage to all the mechanism, even to the extent of softening the rod of the main bell to the point at which it can no longer sustain the weight of the latter, thus causing its loss into the furnace, an accident involving a frightful amount of hard work and serious financial loss due to a shut down of from twelve to forty-eight hours, according to the style of top and the preparedness of the management for such emergencies.

With the gas seal bell normally closed it cuts off by far the greater portion of the leakage of gas and reduces it to an amount whose combustion can do but little harm. I have run a furnace for several days with a huge piece out of the main bell, because the hood and the gas seal being tight the quantity of gas which could escape was unimportant, but it would be impossible to operate a furnace in this condition if the normal position of the gas seal were open, because such a quantity of gas burning continually would destroy the furnace top completely. Even a much smaller leak is a very serious matter under these conditions; the result is that more work is required to keep the furnace top tight enough for safety, and main bells have to be changed more frequently because only small leaks past them can be tolerated.