414 Professor Forbes' s Eleventh Letter on Glaciers. 



ing at East Walden, lieicester, and Nottingham, and before midnight at 

 Southampton and Paris. About 4 a.m. of this day an extraordinary agi- 

 tation of the sea was observed at Penzance Pier by the labourers em- 

 ployed there, the tide being about five hours ebb, and the sea very calm. 

 The water suddenly returned towards the shore, rising between 1 and 2 

 feet, and after an apparent pause, rushed back " like a river," to its for- 

 mer level — the time occupied in the influx and the reflux, including the 

 time the water appeared to be stationary, was about six minutes. The 

 flux and reflux were observed only once. 



In London the thermometer on this day was 89 1° in the shade, and 

 116° in the sun (the latter the maximum for the year), and throughout 

 the previous night it did not descend below 70° ; at Paris it was 90° a 

 great part of the day. At Penzance the nights of the 29th, 30th, and 

 31st of July, were the warmest of the year. On the 30th a terrific 

 thunder-storm, with heavy rain and hail, occurred in Mount's Bay, and 

 throughout Cornwall, and also in Wales and Cumberland, from which 

 time, until after the great storm of the 1st of August, the atmosphere in 

 Cornwall, London, and probably throughout England, was not only very 

 sultry, but highly charged with electricity, whilst violent thunder storms 

 were experienced in various places. 



An earthquake shock was felt along the Rhine on the evening of the 

 29th of July — the moon's first quarter was on the 31st— and the agita- 

 tion of the sea above described (the result perhaps of a submarine shock) 

 happened on the 1st of August. 



In conclusion, I may remark that, with only one exception, all the 

 ahocks of the earth and extraordinary oscillations of the sea in Cornwall 

 during the present century, whose dates are known (and which are eight 

 in number), have happened nearer to the moon's first change or quarter 

 than to any other. The exception is the shock of the 20th of October 

 1837» the day before the moon's last quarter. 



Eleventh Letter on Glaciers ; Addressed to Professor Jameson. 

 (1.) Observations on the Depression of the Glacier Surface. 

 (2.) On the Belative Velocity of the Surface and Bottom of 

 a Glacier. By Professor J. D. FOKBES. With a Plate. 



My dear Sir, — In my Tenth Letter on Glaciers, which you 

 did me the favour to publish lately, a question was discussed 

 respecting the apparent depression of the surface of a glacier. 

 I had already pointed out in the first edition of my Travels, 

 that several causes combine to produce this depression, but 

 that observations were wanting to distinguish them. The 

 causes then enumerated were (if I mistake not) these : — 

 1. The actual waste or melting of the ice at its surface, 



