484 
Proceedings of the Royal Society 
traces of markings, except at the place I have described, from 
which the debris has been removed. Neither could I detect 
markings on the rock faces immediately above the striated sur- 
faces, but these upper faces having been exposed to the weather, 
and never covered by debris, might, though at one time scored, 
gradually lose the markings, while those in the lower portion of the 
hill remained protected by the debris. 
The existence of these markings seems to supply another link in 
connecting the “ crag and tail” formation with glacial action, at 
least in the case of North Berwick Law. From the appearances 
which the removal of this debris have disclosed, we are warranted 
in concluding, that after the passage of the ice-sheet or glacial 
current, the rocky face of the Law, perhaps to its whole height, 
depending on the depth of the abrading agent, was similarly rough- 
polished and scored, that these markings on places exposed to 
atmospheric action have been gradually destroyed, while similar 
markings on the base have been preserved by the covering of 
debris, and may now be seen almost in their original state, if not of 
freshness, as least distinctness of marking. 
If this view be correct, it is likely that by removing similar 
deposits from the base of Stirling Castle, Craigforth, and other 
similar rocks, interesting traces of glacial action in connection with 
the “tails,” which exist at these places would be disclosed. 
4. Laboratory Notes. By Professor Tait. 
a. Photographic Records of the Sparks from a Holtz Machine. 
To determine the cause of the ordinary zig-zag form of electric 
sparks, the author requested Mr Matheson, one of his laboratory 
students well skilled in photographic processes, to take instantaneous 
photographs of the sparks of the Holtz Machine, by means of a 
quartz lens, in hot and cold air alternately. Several of these 
photographs were exhibited, and showed much greater smoothness 
of the track of the spark in heated than in cold air. The zig-zag 
appearance seems to depend upon the presence of combustible 
organic particles in ordinary air, but the experiments are still in 
process. 
