AND LONG-CONTINUED CHANGES OF LOAD ON WROUGHT-IRON GIRDERS. 313 
Deflections produced with permanent weights on hot- and cold-blast cast-iron bars 
4 feet 6 inches between the supports. 
Cold-blast, 
Weight in 
lbs. 
Deflection, 
in 
inches. 
Date of observation. 
Temperature, 
Eahr. 
Hot-blast, 
Weight in 
lbs. 
Deflection, 
inches. 
336 
1-316 
June 23rd, 1838. 
78° 
336 
1-538 
336 
1-308 
April 19th, 1842. 
58° 
336 
1-620 
392 
1-824 
June 23rd, 1838. 
78° 
392 
1-803 
392 
1-828 
April 19th, 1842. 
58° 
392 
1-812 
448 
1-457 
June 23rd, 1838. 
78° 
448 
448 
1-449 
April 19th, 1842. 
58° 
448 
From the above it will be seen that there is no increase in the deflection of the cold- 
blast bar with the 336 lb. load, but a slight increase of - 082 of an inch in the hot- 
blast. With the 392 lbs. there is a slight and progressive increase in both bars, and in 
those with a load of 448 lbs. there is no change but what is due to the difference of 
20° of temperature between the month of June and that of April. As respects the 
load of 448 lbs., it is proper here to observe that the hot-blast bars broke at once with 
that weight, and one of the cold-blast bars also broke after sustaining the load 37 days, 
but whether by accident or from vibration is not determined. It is, however, evident 
from the breaking of the hot-blast bars, and one of the cold-blast, that the load of 
448 lbs. approximated very close on the point of fracture, and that the slightest vibra- 
tion of the floor would break the bar. 
Viewing the subject in this light, it would appear from these experiments that time 
is an element which in a greater or less degree affects the security of materials when 
subjected to long-continued pressure. It may at first sight appear that the cohesive 
powers and the resistance may be so nicely balanced as to neutralize each other, and in 
this state would continue to sustain the load in that condition ad infinitum , provided 
there be no disturbing force to produce derangement of the parts, and thus destroy the 
equilibrium of the opposing forces. This cannot, however, be expected, and I think we 
may reasonably, under ordinary conditions of disturbance, conclude that long-continued 
strain will tend to lessen the cohesive force which unites the particles of matter 
together, and ultimately destroy that power of resistance so strongly exemplified in the 
above experiments. (Vide Report, Transactions of the British Association for 1842.) 
As the object of this inquiry is to ascertain the limit of safety in structures, such as 
railway bridges, subjected to vibration and impact from a rolling load, it may be 
necessary, for the purpose of illustration, to refer to experiments made by the Com- 
mission appointed in 1848 to inquire into the application of iron to railway structures. 
In these inquiries the late Professor Hodgkinson and Professor Willis entered elabo- 
rately into the experimental as well as the mathematical investigation; but the 
experiments which bear more directly upon the present inquiry are those of Captain 
Henry (now Sir Henry) James and Captain Galton, for determining the effects pro- 
2 u 2 
