TKANSACTIONS OF SECTION G. 870 



when it was found that the strength of the link was abo-t IJ time the streng-th 

 of a single section of the iron wire from which it was made. 



The author understands that this chain is not yet marijifactured in this coiintrv, 

 but steps are being taken to introduce tlie machines necessary for the purpose. 

 Specimens of various kinds of chain were exhibited to the Section. 



The author acknowledges the assistance of Mr. W. E. Kerslake, Assistant 

 Lecturer in the Walker Engineering Laboratoiies, ia making the tests and 

 preparing the diagrams. 



TUESDAY, AUGUST 9. 

 The following Papers were read : — 



1. Dredgers. By A. Browk. 



2. Note on Motors used for For/ Signals in the Northern LigJitJiouse Service. 

 By Daviu a. Stevexsox, B.Sc, F.Il.S.E., M.L'st.C.E. 



The paper points out the growing necessity and importanc:' of fog signals on our 

 coast. Details are then gi^■en of the duration of fog in this country, the maxi- 

 mum number of hours in Scotland being .305 hours per aunum. 



The most powerful sound signal in use is stated to ha^e been found by experi- 

 ment and practice to be the ' Siren,' actuated bj^ compressed air. The various 

 motors are then described ; namely, motion of sea by waves and tide, manual 

 labour, clockwork, steam engines, hot-air engines, gas engines, and oil engines. 

 Examples of each are given in detail, and their relative advantages and disadvan- 

 tages discussed. In conclusion the author states in his view the best motor for 

 fog signal purposes yet tried is undoubtedly the oil engine, and concludes by point- 

 ing out how feeble and unreliable at their best are, and must be, fog signals which 

 appeal to the ear, and states that, in his opinion, the solution of the matter will 

 probably be found in the adoption of some such method as that recently proposed 

 by Mr. Charles A. Stevenson, namely, an electric cable or conductor laid down oft' 

 a coast or danger, so as to act on an instrument attached to each vessel, and so 

 warn the sailor of his proximity to it, and therefore to the coast or danger. 



3. On the Progress of the Dioptric Lens as nsfd. in Li.jhthouse Illumination, 

 By Charles A. Stevenson, B.Sc, F.B.S.E., M.Inst.G.E. 



Fresnel, in 1820, devised and constructed a lens for first -order lights of 920 mm. 

 focal distance. It was composed of a central lens, with refracting prisms con- 

 ■centric with it, which gradually decreased in breadth as they I'eceded from the 

 centre. The separate pieces of which these lenses were made up were cemented 

 together and mounted in metallic frames 30 inches square. 



In 1835 the late Mr. Alan Stevenson introduced the Fresnel apparatus into 

 Great Britain. In doing so he made several improvements, one of which was that 

 he increased the height of the lens from 30 to 39 inches. This refractor had eight 

 prisms above and eight prisms below the central lens. Since that time Alan 

 Stevenson's lens was almost universally used until a comparatively recent date, 

 when a revolution in the size of lenses took place. A few years ago inventors were 

 trying to obtain greater power by increasing the diameter and volume of the 

 flames ; but Messrs. Stevenson pointed out, in 1869, that after a certain point an 

 increase of diameter of the luminary not accompanied by a corresponding increase 

 of the radius of the apparatus was a mistake, as the light became exfocal and 

 hence divergent, and that the proper way to secure greater power was to enlarge 

 the diameter of the apparatus. In 1885 they had a lens made to their design of 



