122 ACTION AND MANAGEMENT OF THE

cumference of a circle, by pulling towards him bymeans of an iron hook the nearest iron arm thatmayhappen to be opposite to the door ; this moves thoseboxes which at the commencement of the opera-tion were over the fire-place, towards the coldestpart of the retort, namely, towards the doorwhich is opposite to the fire-place, and a secondseries, or four of the adjacent boxes, are brought inturn into the hottest part of the retort, or over thefire-place, from whence the preceding boxes wereremoved.

When the coal in the second series of boxes hasbeen tw o hours in the hottest part of the retort, itsdecomposition will be completed ; the w orkmantherefore turns the shaft again one-third part ofa circle, and a third series advances in their place,w hile at the same time the first series becomessituated opposite the entry door of the retort, fromwhence they may be withdrawn and exchangedfor an extra set of trays, ready charged with coaland placed on the iron table for that purpose.

In this manner the operation proceeds. One-thirdpart of the whole charge of coal within the retort is