128 SECTIONAL ADDRESSES. 



of air in the moving masses of water, is to make for the free escape of this 

 air and, therefore, for the greater safety of the structure as a whole, though 

 the density and weight of the whole mass containing the larger voids will 

 be less. 



This, of course, is in some measure a disadvantage so far as its resistance 

 to the sea is concerned, and involves large unit masses of rock. In break- 

 water design, therefore, as in most things, it is a question of compromise, 

 and it is found necessary — 



(n) Either to prevent voids entirely where heavy seas are to be 



encountered near high-water level and to a considerable depth 



below it, by means of rectangular blocks built closely, or — 

 {b) To make big irregular masses of artificial rock, such as roughly 



shaped concrete blocks, or — ■ 

 (c) To procure such large rough masses of rock from suitable rock 



stratifications at not too great a distance or expense. 

 As to greater depths of a breakwater, however, say at 45 to 50 feet 

 below the low-water level, where there is no included air possible, the 

 matter of voids and sizes of individual portions of the whole mass of the 

 breakwater bank is not so important. I have already given some 

 instances of the effect of included air between rectangular joints of built 

 masonry and also of the movement by sea action of very massive blocks 

 stacked a little above high-water level at Dover Harbour. I have also 

 roughly described the structure of the great breakwater at Valparaiso in 

 187 feet of water where, at the lower levels, sand alone forms the basis 

 of the biggest breakwater )-et built. This Valparaiso work is an illustra- 

 tion of the varying needs of breakwater construction subject to very 

 heavy seas, and is daring and original in its design, and great credit is 

 due to the Chilean engineers, Senores Davila and Lira, in their advocacy of 

 this novel construction. 



There is another interesting matter to civil engineers in connection 

 with voids, which arises when dredging materials by means of suction 

 pumping. The ease of movement of the material to be pumped and the 

 consequent economy in pumping are largely influenced by the water 

 contained in and surrounding the particles it is desired to move and lift 

 by suction. Mud or clayey material, for example, will allow the water 

 sucked by the powerful pumps to slide over their surfaces, whereas sand 

 and even lumps of rock the size of one's head, if surrounded by water- 

 filled voids, will be moved with relative ease. 



Bacteriological and Entomological Sciences and their 

 Influence on Civil Engineering. 



Some very large and important works could not have been carried out 

 without great loss of life but for the discoveries of Sir Ronald Ross, Sir 

 Patrick Manson, Bruce and others in connection with the disease- 

 carrying powers of certain mosquitoes, and especially the stegomyia and 

 anopheles mosquitoes, which transmit yellow fever and malaria. 



In the early days of De Lesseps' effort to build the Panama Canal 

 the death-rate was very high indeed, and did more to make the first efforts 

 a failure than anything else, unless perhaps finance or economics. I do 



