546 SEC. 12. APPLIED MECHANICS. 



2204. Model of First-class Fixed Dioptric Light. This 

 apparatus consists of a central lenticular band, and an upper and 

 lower set of reflecting prisms. The cylindrical belt, with diagonal 

 joints, and the upper and lower reflecting prisms, were substituted 

 by Mr. Alan Stevenson, in 1836, for the segmented belt and upper 

 and lower silvered mirrors of Fresnel's first-class apparatus. 



(One-fifth of full size.) 



The Commissioners of Northern Lighthouses. 



2206. Model of Mr. Thomas Stevenson's Marine Dyna- 

 mometer, for ascertaining the force of waves during storms. The 

 greatest force recorded in the German Ocean was 3J tons per 

 square foot. Messrs. D. and T. Stevenson. 



2207. Drawing of Storm Curve for Waves, showing 

 the genesis of waves seawards from the windward shore, illus- 

 trative of Mr. Thomas Stevenson's formula h=.\* 5 -v/D, where /*= 

 height of wave in feet, and D=length of fetch in miles. 



Messrs. D. and T. Stevenson. 



2209. Historical Series of published engravings, showing 

 the improvements in lighthouse illumination between 1787 and 

 1876, by the Engineers of the Northern Lighthouse Board. 



The Commissioners of Northern Lighthouses. 



22O9a. Ship's Light, arranged to show a port, starboard, or 

 anchor light. J. S. Starnes, Broad Street, Ratclifr, E. 



South Kensington Museum. 



22O9b. Ship's Lights, port, starboard, and mast head light. 

 Stevens & Sons, Southwark Bridge Road. 



South Kensington Museum. 



XI. MISCELLANEOUS. 



Blenkinsop's Bail and two Chairs. 



F. J. Bramicell, C.E., F.R.S. 



This rail was designed and patented by Blenkinsop in 1811 to meet a diffi- 

 culty propounded by Trevethick previously ; as to the adhesion of the wheels 

 of a locomotive engine to the rails. 



Trevethick proposed that the periphery of the driving wheels of a loco- 

 motive should be made rough by the projection of bolts or cross-grooves, so 

 that the adhesion of the wheels to the rail should be certain. 



Blenkinsop, to meet Trevethick's notion, caused his rail to be made with 

 studs or racks along one side of it, into which a toothed-wheel on his loco- 

 motive was to work. This toothed-wheel was fixed to the driving wheels of 

 the engine, and merely served to gear into the rack on the side of the rail. 



