— 83 — 
lhieal foot in the trusses, floor system and lateral systems, the weight of lumber per 
liueal foot and that of tlie rails, and subtracting from the sum 32 pounds to allow 
for the weights of those portions which rest upon the masonry, and which do not 
affect tli o stresses in tho trusses. This allowance lias been checked for several 
spans of various lengths, and has been found to bo almost constant for all lengths of 
span. , • 
By doubling the dead loads of Table I can be found approximate values for the 
dead loads of double track bridges. Although this method would be a ratlier rough 
a Pl)roxiiuatioii in calculating the weight of iron for a double track bridge from that 
a single track bridge, still it will be found so accurate, as far as dead load is 
concerned, as not to necessitate a re-calculaiion of stresses. This is as one might 
anticipate ； for the truss weights are almost directly proportional to the total loads, 
except when tlie change is so slight as to cause no alteration in the depth of tlic top 
cli?rd and batter brace channels ; tho weights of the lateral systems, iliough not 
wice as great, are considerably greater in double Ilian in single track bridges； for, 
although the total areas opposed to wind pressure arc only slightly greater in tlic 
f ormer than in the latter, yet all the members are longer, and those subjected to 
compression are consequently of larger sectional area ; and the weights of the floor 
system are more than doubled ； for those of all the members except the floor beams 
arc doubled, while those of the latter are increased both by reason of their length and 
h tlic doubled loads thereon. The author lias tested an actual case, and lias found 
that both the weight of iron and the dead load are almost exactly twice as great in a 
double as in a single track bridge of the same span and live load per track. 
From actual measurements of a number of cars tho average area per lineal foot 
of train exposed to wind pressure lias been found to be about eight squaro feet. Tho 
assumed wind pressure of lliirty pounds per square foot on the train agrees with that 
the best American practice : moreover it is as high as economical reasons will 
allow， for it would probably overturn any ordinary train, as the following calculations 
、vill show. 
Let P — pressure per lineal foot of train 
h =： height of centre of pressure above rails 
d = distance between centre lines of rails 
and W = weight of car per foot which will jubt resist the over turiiing 
moment 
then Ph = iWdmi W= 
Tl】e value of h is about seven feet, that of d exactly 8.7 foot tuid that of p two 
hundred and forty pounds. 
Consequently W = - x2 ^ x7 ^ ： 908 potuuls. 
0.7 
Äs the greatest allowable car load per foot is only 1070 pounds, it is highly 
improbable that the average load would reach nine hundred pounds, so that a pres- 
sure of thirty pounds per square foot iu upsettiug a train ou a bridge would destroy 
