178. A bcdy in motion on being brought to a state of rest, exerts a greater force than the same body would if acting by its dead weight alone.

179. A beam resists the application of a moving force or of a load applied suddenly, by yielding until the resisting becomes equal to the straining force in the same manner as a spring, and if the beam is not sufficiently elastic it may break.

180. The power with which a beam resists the impact of a body in motion is termed its ' resilience,' and is simply proportional to the bulk or weight of the beam; thus a beam 10 feet long will support but half as great a pressure without breaking as a beam of the same breadth and depth which is only 5 feet long, but it bears the impulse of a double weight striking against it with a given velocity, and will require that a given body should fall from a double height in order to break it.

The resistance of beams to impact is directly as the product of their strength to their ultimate deflection, and since the strength is as BxD2/L, and the ultimate deflection L2/D, power of resisting impact is as

BxD2/LxL2/D = L2/D = BxDxL, or simply as the solid contents or weight, B, D, and L representing the breadth, depth, and length respectively.*

It has been found by experiment on beams that the deflections are as the square roots of the heights through which the weight falls, or, in other words, the velocity of impact. Mr. Bevan made the following experiments on a beam 18 feet long between the supports, the weight of the beam being 127 lbs., and to bend it 1 inch in the middle required a steady load of 148 lbs. The weight of the falling body which produced the deflections given in the Table was 28 lbs.: -

Table XXVII

Height fallen in inches.

Depression in inches.

Depression as the Square Root of the Fall.

12

1.25

1.25

24

1.66

1.77

36

2.35

2.17

48

2.62

2.5

* Clark's ' Conway and Britannia Tubular Bridges.'

181. The action of the rolling to which a railway bridge is subjected is intermediate, in those cases which occur in practice, between that of an absolutely sudden load and a perfectly gradual one.

In practice the additional strain arising, whether from the sudden application or swift motion of the load, is sufficiently provided for by making the factor of safety for the travelling part of the load about double that for the fixed part.*