, _s_).
Thus the force (_f_) necessary to maintain the motion of the gas in
the pipe is seen to vary (1) as pi_ld_, of which pi is a constant;
(2) as _v^2_, where _v_ = the velocity in feet per hour; and
(3) as _s_. Hence, combining these and deleting the constant pi, it
appears that
_f_ varies as _ldsv^2_.
Now the actuating force is equal to _f_, and is represented by the
difference of pressure at the two ends of the pipe, _i.e._, the
initial pressure, viz., that at the place whence gas is distributed or
issues from a larger pipe will be greater by the quantity _f_ than
the terminal pressure, viz., that at the far end of the pipe where it
branches or narrows to a pipe or pipes of smaller size, or terminates in
a burner. The terminal pressure in the case of service-pipes must be
settled, as mentioned in Chapter II., broadly according to the pressure
at which the burners in use work best, and this is very different in the
case of flat-flame burners for coal-gas and burners for acetylene. The
most suitable pressure for acetylene burners will be referred to later,
but may be taken as equal to p_0 inches head of water. Then, calling the
initial pressure (_i.
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