278 Proceedings of the Royal Society of Edinburgh. [Sess. 
(156) 1912, April 23, 6.10 a.m. 
The shock was felt as a thud (with sound) at Airthrey, and as a slight 
tremor at Alva. 
(157) 1912, April 26, about 4 a.m. 
A shock (intensity 4), felt at Bridge of Allan. 
(158) 1912, May 3, 6.20 a.m. 
A slight tremor, felt at Airthrey. 
(159) 1912, May 3, about 2.15 p.m. 
A slight shock, accompanied by a sound like distant thunder, was felt 
at Alva. 
(160) 1912, May 3, 4.13 p.m. ( Third Principal Earthquake.) 
Intensity, 7 ; centre of isoseismal 6 in lat. 56° 10'*5 N., long. 3° 52'*5 W. ; 
number of records, 91, from 53 places, and 11 negative records from 11 
places (fig. 6). 
Though the disturbed area of this earthquake is less than that of the 
earthquakes of September 21, 1905, and October 20, 1908, the shock within 
the central isoseismal was certainly stronger than that of any other felt 
during the present century. A small increase of intensity would probably 
have brought it within the range of destructive earthquakes. 
On the map of the earthquake are shown four isoseismal lines, corre- 
sponding to intensities 7, 6, 5, and 4. Portions of three of these curves 
are somewhat doubtful owing to insufficient observations, and are indi- 
cated by broken lines. The isoseismal 7 is 8^- miles long, 4f miles wide, 
and 32 square miles in area. The isoseismal 6, which is the most accur- 
ately drawn of the series, is 14 miles long, 8-J- miles wide, and contains an 
area of 92 square miles. Its centre is about 2 miles N.N.W. of Menstrie, 
and the direction of its longer axis is E. 7-|° N. The isoseismal 5 is 19 miles 
long, 12^ miles wide, and 182 square miles in area. The isoseismal 4, which 
may be regarded as the boundary of the disturbed area, is 30 miles long, 
29J miles wide, and contains 605 square miles. The distance between the 
isoseismals 7 and 6 is 2^ miles on the north side and 1^ miles on the south ; 
between the isoseismals 6 and 5, 2-|- miles on the north side and 1^ miles 
on the south ; between the isoseismals 5 and 4, 9 miles on the north side 
and 4J miles on the south. 
Outside the isoseismal 4, the earthquake was felt by persons lying 
down at Dunfermline (17 miles from the centre of the isoseismal 6), 
Glasgow (26 miles), Rutherglen (27^ miles), and Musselburgh (35 miles). 
If the disturbed area were regarded as bounded by an isoseismal of 
