4G 
POPULAR SCIENCE REVIEW. 
and Prussia the quantity of sulphur is larger, the scale being 
saltpetre 7 5 and 12*5 parts each of sulphur and charcoal, while 
in Chinese powder the amount of sulphur is between 14 and 15. 
It is remarkable that in all countries the proportion of saltpetre 
remains about the same. 
The ingredients, being weighed for a charge of 50 lbs., are 
poured into a “ churn.” This is a revolving drum, placed hori- 
zontally, and having within it an axis revolving in a different 
direction from the drum, and furnished with eight rows of pro- 
jecting arms, or “flyers.” So rapid is the action of this appa- 
ratus, that when the charge has been three minutes in the 
revolving churn the ingredients are thoroughly mixed together. 
It is then known as a “ green charge,” and is ready for the 
incorporating mills, the object of which, as the name indicates, 
is to incorporate the materials, or to make the mixture so 
intimate that a new substance is produced, namely, gunpowder. 
The incorporating-houses at Waltham contain at present 
thirty-two separate mills. Each mill consists of a pair of run- 
ners, coupled together by a strong axle. This axle rests in the 
socket of an upright shaft, which, passing down through the 
mill-bed, is connected by bevel-wheels with a revolving hori- 
zontal shaft, driven by steam or water-power. The runners are 
either of black Derbyshire limestone or of iron, and weigh from 
three and a-half to four tons. Iron runners are now generally 
used, and their size varies from three and a-half to seven feet in 
diameter. The mill-bed, a large circular vat with a flat bottom 
and sloping sides, is of stone or metal, according to the ma- 
terial of the runners. On this bed 50 lbs. of the green charge 
is spread out and moistened with water, and the mill is then 
set going. The length of time required for the incorporation 
of the powder varies according to the use to which it is to be 
applied. Thus cannon powder is left under the mills for three 
hours ; while for rifle powder, which requires a closer incorpo- 
ration on account of its more rapid action, the time is five 
hours. The power of a gunpowder factory is measured by the 
number of pairs of runners it possesses, for as the law allows no 
more than 50 lbs. to be placed in any mill at one time, the 
amount which can be incorporated in a year is easily calculated. 
A pair of iron runners, driven by steam and working day and 
night, will incorporate in a year nearly 100,000 lbs. of cannon 
powder, or about half that quantity of rifle powder. 
This part of the process is more dangerous than any other, 
and explosions in the incorporating-mills are very frequent. 
The houses are built of light planking, nailed on a strong frame- 
work, so as to diminish the force of the explosion by yielding 
easily before it. The men are forbidden to remain in them 
while the mill is in motion, and a very simple arrangement has 
