I. Introduction On April 13th, 1902, a slight earthquake occurred in the north of Lincolnshire. Its intensity was not more than 4, and the area disturbed by it included about 600 square miles. The shock was of the simplest character, lasting on an average for 4 seconds, and consisting of a single series of vibrations, which increased in intensity to a maximum and then died away. Though of little consequence when considered as an isolated event, the shock derives interest from its probable connection with the much stronger twin-earthquake which occurred in the same district three years later, on April 23rd, 1905. On this day, there were two undoubted earthquakes, namely:— ( a ) About 1.30 a.m. ( b ) 1.37 a.m. (principal earthquake). In addition to these, several disturbances of uncertain seismic origin are reported:— April 23rd, about 1.39 a.m.: Barton-on-Humber.—A very slight shock, similar to that of the principal earthquake, and accompanied by a sound like distant thunder. April 23rd, about 2.5 a.m.: Farnsfield and Normanton-on-Trent.—A slight shock, without any attendant sound, was felt at these places, both of which are more than 15 miles from the principal epicentre. At Farndon (near Newark) subterranean noises are said to have been heard on several occasions, especially on April 27th, at 11.30 p.m., and on April 29th, about midnight. A slight shock was felt at Epworth, and a rumbling sound was heard at Norton. Epworth lies between the two portions of the isoseismal 7 of the