After the seed is crushed in this way, it is passed under a pair of edge stones.  These stones weigh about seven tons, are 7 feet 6 inches in diameter and 17 inches broad, and make seventeen revolutions a minute.  If of good quality, they will not require to be faced more than once in three years, and they will last from fifteen to twenty.  They are fitted with two scrapers, one for raking the seed between the stones, the other for raking it off at the proper period.  One pair of stones will grind seed sufficient for two double hydraulic presses, and the operation occupies about twenty-five minutes.  The seed is now crushed and ground, but before it is passed on to the press it is transferred to the heating-kettle.

The heating-kettle is composed of two cylindrical castings, one fitting loosely into the other, so that a space is left between them for a free circulation of steam all round both the sides and bottom of the interior vessel.  The internal casting is again divided horizontally into two partitions, one above the other therefore, by two plates, between which also there is a space left for the admission and circulation of steam; and a communication is kept up between the upper compartment and the under by means of a stripping valve.  Besides this, there is a communication from the internal kettle through the external one, and also a shaft passes between the two horizontal parts to give motion to the stirrer, which revolves thirty-six times a minute.  A cover encloses the top, and it is through this the vessel is charged.  The upper portion is filled first, where the contents introduced are allowed to remain ten or fifteen minutes, after which the valve is opened and the whole falls into the lower kettle, where it is kept till wanted.  The seed is then taken away from the lower kettle by an opening, and bestowed in bags of sufficient size to make a cake of 8 lbs. weight after the oil is pressed out of it.  Indeed, the compartments of the heating-kettle are of a size to contain enough to charge one side of a hydraulic press.  These, therefore, are so constructed as to render the operation continuous, the upper one being discharged into the under as soon as its contents are withdrawn to the press.  The seed is heated to the temperature of 170 degrees Fahr., when it is drawn off and placed in the bags.

In another form of kettle the seed is heated on a hot hearth, and on the top of the hearth is a loose ring, within which a spindle revolves to stir the seed.  After the requisite temperature has been reached, the ring is raised and the seed swept into the bags, which are made of horse-hair.  There is great loss of heat in this method, however, as the seed is exposed to the atmosphere, which of course cools it.