TRANSACTIONS OF THE SECTIONS. 219 



On the Plymouth Breakwater. By R. C. Townsend, Superintendent. 



The author offered a few remarks on the damage done during the storm of 7th and 

 8th January 1807 to the western arm of the Breakwater, the most vulnerable part 

 of the whole structure — first observing that during a South or S.W. gale, and 

 about an hour and a half before or after low-water spring tides, the heaviest seas 

 culminate and strike the foot of the south or sea slope with destructive force, very 

 much exceeding that on any other portion of the work. 



It would seem that each wave increases in volume from the Draystone Buoy, off 

 Penlee Point, running in line with the Knap Buoy and Panther Shoal, finally 

 impinging on the slope of the west arm of the Breakwater, where, especially in the 

 winter months, it invariably dislodges from the fore shore rough limestone blocks 

 varying in weight from 5 to 10 tons, carrying them over the work and depositing 

 them on the inner slope. But in the case of the storm referred to (1867), not only 

 were large rough blocks swept over, but the masonry forming the granite band 



ftrotecting the foot of the slope was upheaved and carried away for a considerable 

 ength, leaving the rough hearting of the work exposed to future assault. The 

 primary cause of weakness I traced to the subsidence of the foundation below the 

 granite band, leaving cavities of considerable length and depth open to hydrostatic 

 influences. The author thinks that had the granite band been seated a few feet 

 lower in the work, and the stones of greater weight, the damage done, amount- 

 ing to over £20,000, might not have been so extensive. The experience thus 

 gained induced him to recommend the Director of Works (now Sir Alexander Clerk, 

 B.E.) to adopt for this especial arm of the breakwater a repair of a more massive 

 character, — that is to say, to carry the new buttress from 3 feet below low-water 

 mark up the slope in a solid mass to a level of about half-tide, the face of this work 

 being of large dressed granite blocks, with dovetailed joints and sunk beds, banded 

 horizontally and vertically, dowels and cramps being introduced in the two upper 

 courses. The new work, thus repaired, stands admirably, and has so far fulfilled all his 

 anticipations. In addition to the stability given to this arm of the Breakwater, the 

 vertical face (in steps) forming the new buttress prevents travelling limestone 

 boulders being lifted and carried over the works ; in fact the accumulation of detained 

 stone at the base of the new work proves to be a valuable adjunct to the strength 

 of this arm of the Breakwater. 



As an example of the heavy waves that occasionally strike the work during a S.S .W. 

 gale, the author mentioned the displacement of two experimental concrete blocks, 

 weighing 24 tons each, and one limestone block weighing 35 tons, deposited 60 

 feet from the toe of the Breakwater and spaced about 30 feet apart. La a storm 

 some eight years ago the two concrete blocks were capsized and in one tide swept 

 completely over the Breakwater and deposited on an inner slope. The limestone 

 block was driven home to the granite band and joined fast amidst other stones. 



As an experiment, the author's predecessor, 13 years ago, deposited on the sea-slope 

 of the Breakwater, not far from the return angle of west arm, a small concrete block 

 (Portland cement), with a view to test its powers of resistance to oceanic influence. 

 During this time the concrete cube has been dislodged more than once by travelling 

 stones in a gale, but the author has again restored it to its original position, halfway 

 down the slope. As it stands now, it appears but little reduced by abrasion, and 

 forms an interesting sample of what good concrete will withstand. 



The whole of the Breakwater is now in excellent condition and may not require 

 heavy repairs for years to come. 



On Electric Block Telegraphs. By P. H. Varley 



The Government Establishments of Plymouth and Neighbourhood, 

 By R. N. Worth. 



17* 



