572 TRANSACTIONS OF SECTION G. 
The value of the rotational hysteresis torque was then found experimentally 
by means of ‘running down’ curves, for several values of the frequency of the 
stator current. It was found that for very low frequencies (four cycles per 
second) the experimentally determined rotational hysteresis torque approached 
closely to the values deduced from the measurements with direct current in the 
stator winding. At higher frequencies the hysteresis torque for a given alter- 
nating current became gradually less until a frequency of about twenty cycles 
per second was reached, when it remained constant over a large range of fre- 
quencies (twenty to fifty cycles per second). 
6. Rescue Apparatus for Coal Mines. By F. Ret. 
7. Experiments on the Weathering of Portland Stone. 
By Dr. J. 8S. Owens, F.G.S., A.M.Inst.C.E. 
This investigation was initiated by Dr. Des Voeux and is being made for 
the Coal Smoke Abatement Society. Its object is to find the connection between 
smoke and the well-known deterioration by sulphating of stones containing a 
large proportion of calcium or magnesium carbonate when exposed to city air. 
The effect of weathering upon the strength of such stones in tension and 
compression was measured. 
The stones were subjected to the following conditions for periods varying 
from 105 to 687 days :— 
(a) Broken without exposure. 
(6) Broken after exposure to natural conditions out of doors in the country 
and in London. 
) Broken after having been kept indoors in the country and in London. 
Broken after having been exposed, while embedded in soot, to country 
and London air. 
The experiments are still going on; four series of stones up to the present 
have been broken, and the results are as follows :— 
Series 1.—Thirty-nine Portland stone briquettes, cut without reference to 
their bedding planes, were broken in tension. All these stones increased in 
strength, some more than others, thus confusing the issue; but embedding in 
soot had a distinctly injurious effect on the strength. 
Series 2.—Twenty-nine similar to Series 1 were broken in tension. The per- 
centage of water present was noted, as its presence influenced the strength 
adversely. Stones exposed in London showed a less resistance than those exposed 
in the country. 
Series 3.—Highteen stones broken in tension showed that the presence of 
43 per cent, water—that is, sufficient to saturate the stones—reduced the strength 
by about 40 per cent. The stones being broken in tension this loss of strength 
could not be due to any form of bursting pressure by the water, as might occur 
with a saturated stone under compression, The results also show that in calcula- 
tions as to the strength of structures built of stone the amount of water present 
should be taken into account. 
Series 4.—Twenty-five 1-inch cubes of Portland stone were broken in com- 
pression. All these were dried before breaking, and the results show in all cases 
that exposure to London air, or embedding in soot, caused considerable loss of 
strength, when compared with stones exposed under normal conditions in the 
country. The compression tests were made in a Wicksteed machine, cardboard 
packing-pieces being placed between the specimens and the compression plates, 
and the loads were applied at a uniform rate. 
The results obtained in each series are promising; but they indicate that 
further tests dealing with a larger number of specimens in each group are 
desirable. 
