53 HARBOUR CONSTRUCTION. 



comparing it with the height of a column of mercury which it 

 will balance. Hence, if we say the barometer stands at 30'20, 

 we mean that the pressure of the atmosphere is such that it 

 will just balance a column of mercury 30'20 inches in height. 



The weight of mercury is about 13J times that of sea- water; 

 therefore a local rise or fall of the barometer amounting to, say, 

 one inch, would represent a depression or elevation, as the case 

 might be, of the surface of the ocean at that place to the extent 

 of about 13 J inches. 



The effect of atmospheric pressure in various places is, how- 

 ever, not uniform, being affected by local conditions. 



On this point Airy writes l 



" We cannot do better than refer the reader to Mr. Lubbock's 

 paper, Phil. Trans., 1837, p. 97, etc. It appears there that in a 

 set of observations considered by Mr. Lubbock, the unexplained 

 fluctuations of the tide correspond precisely to those of the barometer; 

 and Mr. Lubbock has laid it down as a rule that a rise of 1 inch 

 in the barometer causes a depression in the height of high water 

 amounting to 7 inches at London, and to 11 inches at Liverpool. 

 Mr. Bunt also, in the " Eleventh Report of the British Association," 

 p. 31, has discussed with great skill the effect of the barometer on 

 the tides at Bristol, and has shown that a rise of 1 inch of barometer 

 produces a depression of 13'4 inches." 



A very little reflection will show to what a large extent the 

 proportions, and consequently the cost, of sea-works are affected 

 by range of tide. 



Let us assume a vertical breakwater to be situated in a tide- 

 less sea, where the depth of water is 20 feet, the width of such 

 breakwater being 30 feet, and its height 10 feet above the water- 

 line. Such a breakwater would contain 100 cubic yards of 

 material per lineal yard. 



A similarly proportioned structure in the same depth at low 

 water, but where the rise of tide was 10 feet, would contain 

 nearly 178 cubic yards of material per yard forward. 



The quantity of material in a pell-mell mound breakwater, 

 constructed of concrete blocks, and having a top width of, say, 

 30 feet, with a slope on the seaward side of 1J to 1, and on the 

 harbour side of 1 to 1, would, under similar conditions, be 

 increased by 58 per cent. 



1 " Tides and Waves," p. 390. 



