542 Progress in Science. [Ocftober, 



left in the eastern end of the breakwater. The estimate for the breakwater 

 alone is, according to this design, ^850,000, and the maximum time of con- 

 struction eight years. Colonel Clarke has, however, proposed an alternative 

 design, in which the railway company's water station is retained in a modified 

 form. The proposed extension of the Admiralty Pier eastwards for 500 feet 

 is retained, and then comes an opening of 600 feet. The breakwater then 

 commences, and turns eastwards for about 2000 feet, when it turns by a curve, 

 and takes a northerly direction to the shore, which it joins about 150 feet 

 eastward of the Castle Jetty. The eastern entrance is omitted, as owing to 

 the currents at that point it would in all probability silt up. The area enclosed 

 is about 350 acres to low-water line. In the space left between the Castle 

 Jetty and the eastern wall of the breakwater, Colonel Clarke proposes to 

 build a small dockyard. From the Castle Jetty, along the whole coast line 

 to the entrance of the present harbour, Colonel Clarke further proposes in the 

 future to have a commercial quay and boulevard with trees and promenades. 

 The whole matter is in the hands of the Board of Trade, and a sum of 

 ;£io,ooo was voted last session for carrying out the work according to Colonel 

 Clarke's alternative design. The breakwater will be formed of concrete 

 blocks, with the intervening space filled in with fluid concrete to a level of 

 about 16 feet below low water. From that point and up to 3 feet above low 

 water, concrete blocks alone will be used. From the last-named level to the 

 top of the work, which is 6 feet above high water, the structure will be carried 

 out in concrete, which will be put in between tides. The modification pro- 

 posed by Colonel Clarke will admit of the completion of the work in five 

 years from the time of their actual commencement. 



Bow and Stem Screw Ships. — A paper on this subject was read on the 

 16th of June last, before the Royal United Service Institution, by Mr. Robert 

 Griffiths, whose name is so well known in connection with the subject of 

 screw propulsion. In order to avoid some of the difficulties and dangers that 

 now attend screw ships, and also to improve their speed, it occurred to Mr. 

 Griffiths that if in addition to the screw at the stern another propeller were 

 applied in the bow of a steamship, both screws being placed in tunnels formed 

 in the side of the ship so as to be protected from coming in contact with such 

 objects as a ship's anchor or cable, it would be the means of avoiding a great 

 many of the difficulties and dangers now attendant upon screw ships. From 

 experiment it was ascertained that the best arrangement was that wherein 

 the water from the bow screw was delivered underneath the ship, and water 

 for the stern screw was taken also from underneath, so that both ends of the 

 ship were made the same below the water-line. In this case the bow screw 

 itself gave a better result in consequence of the water discharged from the 

 screw meeting with a greater resistance, giving the same effect as is now pro- 

 duced by lowering the screw, and thereby obtaining a deeper immersion of 

 the blades. Another great advantage may be obtained by this method of 

 placing the screw, and that is, that the screw may be made to discharge any 

 bilge water or any great leakage that may happen to take place in the vessel. 

 As a result of trials made with a small vessel thus fitted with bow and stern 

 screws, Mr. Griffiths asserts that besides numerous other advantages from 

 this method of disposing the screws, 20 per cent less power is required by the 

 screw when working in the tunnel to obtain the same speed than when 

 working in the ordinary way. 



Rock Drills. — A paper on the Diamond Rock Drill was recently read before 

 the Iron and Steel Institute at Liege by Major Beaumont. The diamond 

 drill is in principle quite distinct from any other system of holing rock, and 

 works by rotation without striking a blow. Its action is rather that of 

 abrading than cutting, and the effect is produced by the sheer difference in 

 hardness between the diamond and the rock it is operating upon. The 

 wonderfully resisting power of the diamond enables machinery of the simplest 

 and most ordinary character to be used, and thus avoids those special difficulties 

 that the mechanic must face when he is driven to utilise a large power in the 

 production of percussive action ; moreover, machinery can be applied in 

 places where a reciprocating motion, if admissible at all, would present 



