rule given in the last answer 606.3 x 10 = 6063, the square root of which
is 78 nearly, and 78 x 4.01 = 312.78, the velocity of the rim in feet per
second at the moment of rupture.

29. _Q._--What is the greatest velocity at which it is safe to drive a cast
iron fly-wheel?

_A._--If we take 2,000 lbs. as the utmost strain per square inch to which
cast iron can be permanently subjected with safety; then, by a similar
process to that just explained, we have 4,000 lbs./49.48 = 80.8 which
multiplied by 10 = 808, the square root of which is 28.4, and 28.4 x 4.01 =
113.884, the velocity of the rim in feet per second, which may be
considered as the highest consistent with safety. Indeed, this limit should
not be approached in practice on account of the risks of fracture from
weakness or imperfections in the metal.

30. _Q._--What is the velocity at which the wheels of railway trains may
run if we take 4,000 lbs. per square inch as the greatest strain to which
malleable iron should be subjected?

_A._--The weight of a malleable iron rim of one square inch sectional area
and 7 feet diameter is 21.991 feet x 3.4 lbs. = 74.76, one half of which is
37.4 lbs. Then by the same process as before, 8,000/37.4 = 213.9, the
centrifugal force in terms of the weight: 213.9 x 7, the diameter of the
wheel = 1497.3, the square root of which, 38.3 x 4.01 = 155.187 feet per
second, the highest velocity of the rims of railway carriage wheels that is
consistent with safety. 155.187 feet per second is equivalent to 105.8
miles an hour. As 4,000 lbs. per square inch of sectional area is the
utmost strain to which iron should be exposed in machinery, railway wheels
can scarcely be considered safe at speed even considerably under 100 miles