what may be called a virtual summit.
It is better to have too many than too few, as accumulations of air may
cause an enormous diminution in the quantity of water delivered. In all
depressions discharge valves should be placed for emptying the pipes
when desired, and for letting off the sediment which accumulates at such
points. Automatic valves are frequently placed at suitable distances for
cutting off the supply in case of a burst. At the inlet mouth of the
pipe they may depend for their action on the sudden lowering of the
water (due to a burst in the pipe) in the chamber from which they draw
their supply, causing a float to sink and set the closing arrangement in
motion. Those on the line of main are started by the increased velocity
in the water, caused by the burst on the pipe at a lower level. The
water, when thus accelerated, is able to move a disk hung in the pipe at
the end of a lever and weighted so as to resist the normal velocity;
this lever releases a catch, and a door is then gradually revolved by
weights until it entirely closes the pipe. Reflux valves on the
ascending leg of a siphon prevent water from flowing back in case of a
burst below them; they have doors hung on hinges, opening only in the
normal direction of flow. Due allowance must be made, in the amount of
head allotted to a pipe, for any head which may be absorbed by such
mechanical arrangements as those described where they offer opposition
to the flow of the water. These large mains require most careful and
gradual filling with water, and constant attention must be given to the
air-valves to see that the gutta-percha balls do not wedge themselves in
the openings. A large mass of water, having a considerable velocity, may
cause a great many bursts by water-ramming, due to the admission of the
water at too great a speed. In places where iron is absent and timber
plentiful, as in some parts of America, pipes, even of large diameter
and in the most important cases, are sometimes made of wooden staves
hooped with iron. A description of two of these will be found below.

Thirlmere.

The _Thirlmere Aqueduct_ is capable of conveying 50,000,000 gallons a
day from Thirlmere, in the English lake district, to Manchester. The
total length of 96 m. is made up of 14 m. of tunnels, 37 m. of
cut-and-cover, and 45 m. of cast-iron pipes, five rows of the latter
being required. The tunnels where lined, and the cut-and-cover, are
formed of