d
be the maximum) the fermenting vessels are fitted with "attemperators,"
_i.e._ a system of pipes through which cold water may be run.

_Cleansing_.--In England the methods of applying the top fermentation
system may be classified as follows: (A) _The Cleansing System_: (a)
Skimming System, (b) Dropping System (pontos or ordinary dropping system),
(c) Burton Union System. (B) _The Yorkshire Stone Square System_.

[Illustration: FIG. 4.--Fermenting Round.
A, Skimmer; B, Parachute; C, Attemperator.]

(A) In (a) the _Skimming System_ the fermentation from start to finish
takes place in wooden vessels (termed "squares" or "rounds"), fitted with
an attemperator and a parachute or other similar skimming device for
removing or "skimming" the yeast at the end of the fermentation (fig. 4).
The principle of (b) the _Dropping System_ is that the beer undergoes only
the main fermentation in the "round" or "square," and is then dropped down
into a second vessel or vessels, in which fermentation and cleansing are
completed. The _ponto_ system of dropping, which is now somewhat
old-fashioned, consists in discharging the beer into a series of vat-like
vessels, fitted with a peculiarly-shaped overflow lip. The yeast works its
way out of the vessel over the lip, and then flows into a gutter and is
collected. The pontos are kept filled with beer by means of a vessel placed
at a higher level. In the _ordinary_ dropping system the partly fermented
beer is let down from the "squares" and "rounds" into large vessels, termed
dropping or skimming "backs." These are fitted with attemperators, and
parachutes for the removal of yeast, in much the same way as in the
skimming system. As a rule the parachute covers the whole width of the
back. (c) The _Burton Union System_ is really an improved ponto system. A
series of casks, supplied with beer at the cleansing stage from a feed
vessel, are mounted so that they may rotate axially. Each cask is fitted
with an attemperator, a pipe and cock at the base for the removal of the
finished beer and "bottoms," and lastly with a swan neck fitting through a
bung-hole and commanding a common gutter. This system yields excellent
results for certain classes of beers, and many Burton brewers think it is
essential for obtaining [v.04 p.0511] the Burton character. Fig. 6 (Plate
II.) shows the process in operation in Messrs Allsopp's brewery.

(B) _The Stone Square System_, which is only used to a certain extent
(