481 
of Edinburgh, Session 1874-75. 
the forms of both intrados and extrados, and at the same time are 
able to satisfy punctiliously the conditions of equilibrium by pro- 
perly adjusting the slope of the inner surface of the arch stone. 
The computation needed for this adjustment is simple and obvious. 
The builder, however, would scarcely think it worth his while to 
cut the stones truly to the shape so found; he would often prefer 
the usual rough-hewn surface and the great cohesion which that 
roughness gives ; and will probably rest contented with a test for 
safety, which test may be found in the very simple law, that the 
difference between the logarithms of the tangents of the inclina- 
tions at the two proximate points of the arch should always differ 
by more than the logarithms of the loads imposed between those 
points and the vertex of the structure. 
2. On the Application of Angstrom’s Method to the Conduc- 
tivity of Wood. By C. G. Knott and A. Macfarlane. 
Communicated by Professor Tait. 
This was an account of a series of experiments made in the 
Natural Philosophy Laboratory of the University, to test the appli- 
cability of Angstrom’s method of periodic variations of temperature 
to the determination of low conductivity. The wood was cut into 
discs of a standard thickness, and these were very tightly secured 
together, after the interposition of copper-iron thermo-electric junc- 
tions (of very fine wire). One series of discs was cut parallel, the 
other perpendicular, to the fibre. The results were obtained very 
easily, and accorded satisfactorily with those obtained by more 
laborious methods. 
3. Notice of Striated Kock Surfaces on North Berwick Law. 
By David Stevenson, V.P.R.S.E., Civil Engineer. 
The well marked “crag and tail” formation of North Berwick 
Law has long been appealed to by geologists as a striking example 
of the effects of those mysterious glacial currents, which at some 
time have wrought such changes on the surface of the globe. The 
Law presents, as is well known, a comparatively bold face, or crag 
