487 
of Edinburgh, Session 1881—82. 
the compacting of the loose material. The moisture among it and 
the first snow that falls on its surface freeze. And thus a thoroughly 
compact moss is provided to resist snow-slips and to permit the 
water, from the melting snow when the thaw comes, to flow over it 
into the courses which feed the mountain torrents. 
9. A careful examination of the deposits within this area, so far 
as they have any hearing on former Arctic conditions of climate and 
a glacial surface, begets the belief that the hulk of the phenomena 
may ultimately find their explanation in the recognition of two 
movements — one from the west, north-west, or north-north-west, 
inwards to the Ben Nevis range, and another, subsequently, 
outwards from Ben Nevis as a centre. 
3. Bemarks on Dielectric Strength. By Professor Chrystal. 
The phenomena accompanying the disruptive discharge of electri- 
city are, in the present state of electric science, among the most inter- 
esting known, because they are the least understood, and, so far as 
we know, the least concordant with our preconceived ideas. The 
simplest way of representing the facts is to imagine with Paraday 
that the non-conducting medium, or dielectric, between two charged 
conductors is the seat of mechanical stress, consisting of tension 
along, and pressure perpendicular to, the lines of force. The rupture 
of the dielectric may then he conceived as a phenomenon precisely 
analogous to the rupture of an elastic body under stress. We are thus 
led to the conclusion that the commencement of the rupture happens 
at that particular point where the tension first reaches a certain value, 
called the breaking tension or dielectric strength, which depends 
merely on the material of the dielectric, and on its physical condition 
at the time being. The main thing in any experiment on dielectric 
strength is to know the tension at the point where the rupture begins. 
According to Maxwell’s rendering of Faraday’s theory, the tension 
is ^R 2 , where R is the resultant electric force and K the specific 
07T 
inductive capacity of the dielectric. If the dielectric he uniform, 
R will he a maximum at the surface of some conductor. If then cr 
he the surface density, since KR = 47rcr, we have at the point of 
rupture, 
