124 
Journal of the Mitchell Society [. November 
sets are 6 ft. 3 inches high in the clear and 5 feet across in 
the clear. The posts are, therefore, 6 ft. 3 inches long be- 
tween the shoulders and have a 5 x 5 inch tenon inches 
long at each end; the caps are 5 ft. 3 inches between shoul- 
ders and have a 5x5 inch tenon 2}£ inches long at each end; 
the ties are 5 feet between shoulders and have a tenon 5x8 
inches and 1% inches long at each end. 
The size of the timbers will of course vary with the weight 
to be sustained. This style and proportions of framing are 
very good for oak timbers; but for pine, which crushes so 
easily across the grain, it is better to have the ends of the 
post tenons to touch each other. The light timbers are of 
course cheaper and much more easily handled. As the stopes 
get large, they are more or less completely filled with waste 
rock which is usually obtained in mining and would other- 
wise have to be hoisted out in working the usually underhand 
stopes. This filling also holds the posts in position and 
helps to prevent them from buckling or “jack-knifing” if any 
one timber yields, which might otherwise endanger the whole 
system. Since most of the pressure is downward, as soon as 
the ore is blasted away to make room for a new set, all the 
sets below are relieved and tend to come back to their origin- 
al position. Thus, even light timbers will hold very well if 
the stope is worked rapidly enough. 
In the Union Copper mine the square sets were founded 
upon a platform built upon the old solid looking truss sets. 
As soon as a heavy load came upon them the trusses buckled 
sidewise and everything caved in. Anew foundation was then 
made upon reinforced stulls and there has been no trouble 
since. Mr. Griswold is starting a new lot of square sets in 
a large open chamber just above the fourth level and he will 
thus be able to work out easily all the ore left above, espec- 
ially a big pillar that remains between the first and second 
levels. 
A new set can be added in any position at any time with- 
out disturbing the adjoining timbers and the old timbers can 
easily be supported by temporary props while making room 
