620 The Trials of Grist Milts and Disintegrators at Plymouth. 
(presumably a bolt-head) had found its way in with the feed, 
and had so damaged the machine as to render more or less 
extensive repairs needful. The experience of less than half an 
hour's trial had made it evident that the grid is the " weak place" 
in the " Collision" mill, inasmuch as, being the most expensive 
part of the construction, and always under bombardment of the 
severest kind, it is peculiarly liable to costly damage. 
It must here be remarked that the " Devil " received several 
pieces of iron with his bones, but these were all apparently re- 
jected by the attendant, who, however, stopped once for a few 
seconds to open his discs, saying that he knew by the sound 
that a piece of metal was within ; whether this was so or not, 
no damage was done. 
The effect of the bolt-head upon the " Collision " mill was 
remarkable and instructive. One of the beaters was bent and 
cracked, one of the steel grids was much deformed, and rendered 
useless without a smith's repair, involving unmaking and re- 
making the " ladder," while the driving spindle itself was bent. 
Now bones are never free from iron, and while careful picking 
cannot insure their becoming so, electro-magnetic screening is, 
at present, only a scheme. The question therefore arises, "Is it 
not desirable that machines dealing with bones or town refuse, 
both of which always contain iron, should break, if they break 
at all, in the cheapest place ? " Iron, entering the " Devil " 
disintegrator, comes first into collision, even when grinding fine, 
with the massive teeth surrounding the inner circumference of 
the grinding ring — teeth whose great length, and consequent 
strength, in the direction of the blow, give obstructions a 
chance of merely locking the rings and throwing the strap off 
the pulley. But in the case of the " Collision " mill, iron, 
entering with the feed, is flung with a speed of 300 feet per 
second directly against the grid, which, when the' mill is 
grinding fine, must necessarily be a comparatively frail struc- 
ture, yet all the more expensive because frail, since it consists 
of more and finer steel "rungs" than the coarser grids. The 
Judges did not therefore regret an accident which, while it was 
apparently a misfortune for the " Collision " mill, was, generally, 
an instructive event. 
Later in the same day, the machine, having been repaired, 
was restarted ; but, in the meanwhile, 2| cwt. of bones had 
been ground, before the accident occurred, at the rate of 
10 cwt. per hour (see Table VI.) and at a considerable expen- 
diture of power. A quantity of bones, equal to one-sixth of the 
material ground, was rejected by the feeder, who, indeed, threw 
out every large piece, together with any hoof or horn that 
