1915-16.] The Ochil Earthquakes of the Years 1900-1914. 261 
82 square miles in area. Towards the north-east, the course of this curve 
is uncertain. The isoseismal 5, which is determined by a much larger 
number of observations, is 20J miles long, 14 miles wide, and 227 square 
miles in area. Its centre is 3 miles N. 25° E. of Menstrie, and its longer 
axis is directed E. 29° N. The distance between the isoseismals 6 and 5 is 
3J miles on the north side and 2f miles on the south. The isoseismal 4 is 
33 \ miles long, 26^ miles wide, and 700 square miles in area. Its longer 
Fig. 3. — Earthquake of Sept. 21, 1905. 
axis is directed E. 27° N. The distance between the isoseismals 5 and 4 is 
7J miles on the north side and 5 miles on the south. Outside this 
isoseismal, the shock was felt at St Fillans and Balmeanach, 3 and 4| 
miles to the north-west, and at Falkirk, 1 mile to the south. The 
disturbed area must therefore contain about 1000 square miles. 
The shock was similar to its predecessors, except that, near the epicentre, 
the principal vibrations were preceded, as well as followed, by tremors. 
The mean of 54 estimates of the duration of the shock is 3’4 seconds. The 
sound-area is co-extensive with the disturbed area. The percentage of 
audibility is 84 for the whole area, 89 within the isoseismal 6, 82 between 
the isoseismal lines 6 and 5, and 79 between the isoseismals 5 and 4. 
