of Edinburgh , Session 1878-79. 
501 
per hour, prevailed, after which the force of the wind rapidly- 
increased, shifting to W.N.W. until about 2 o’clock when it became 
a perfect hurricane, accompanied by a storm of snow and sleet. 
The velocity of the wind must have been betwixt 40 and 50 miles 
per hour, and partly from heavy black clouds covering the sky, and 
partly from the dense sheet of snow which was drifting along, every- 
thing became obscured, 
From the point where I stood (at the door of my house, the Old 
Observatory, Calton Hill) there appeared two currents of snow drift, 
one on each side, which meeting each other about 20 feet in front of 
me, after a severe struggle coalesced and shot upwards with excessive 
velocity slantingly, and towards E.S.E. As that upshot was going 
or gone, but when the storm was still at its maximum, a loud crash 
was heard, and a vivid flash of light simultaneously seen, much as 
one may suppose would be the effect of the bursting of a bomb-shell 
within a few feet of you. 
Almost immediately after this the storm began to abate, and the 
remainder of the day was comparatively calm. Altogether the 
storm did not last above ten minutes, and during that time the 
barometer fell y^-ths of an inch, but rose again immediately after- 
wards to its former height. 
4. On the Colouring of Maps. By Professor Tait. 
(Abstract.) 
Some years ago, while I was still working at knots, Professor 
Cayley told me of De Morgan’s statement that four colours had been 
found by experience to be sufficient for the purpose of completely 
distinguishing from one another the various districts on a map. 
I had previously shown that if an even number of boundaries 
meet at each point on a diagram, two colours (as on a chess-board) 
will suffice for the purpose. But in a map, boundaries usually meet 
in threes. 
I replied to Professor Cayley that I thought the proof might be 
made to depend upon the obvious proposition that not more than 
four points in a plane can be joined two and two by non-intersecting 
lines. Here points were made to stand for districts. When two such 
points are joined by a line they must have different colour-titles. I 
