1873] WRITINGS OF JOSEPH HENRY. 367 



he should make, from time to time, inspections of light-houses 

 at various points on the coast. The inspector of each district 

 is required to visit, at stated intervals, each light-house within 

 his jurisdiction after completion by the engineers, to correct 

 any delinquencies on the part of the keepers, and to supply oil 

 and other materials necessary to the efficient maintenance of 

 the signals, and finally to inform the engineer as to any re- 

 pairs which may be required. The district engineers, as well 

 as the engineer officers of the board, find full employment for 

 all the theoretical knowledge and practical skill they possess 

 in the surveys of new sites, making studies for the con- 

 struction of new permanent aids to navigation (many of 

 them on submarine sites in exposed positions), in planning 

 and rearing the towers, and in fitting up the lenticular 

 apparatus. - - - - 



It has been thought that the light-house system is of a 

 practical character, and therefore does not require the aid of 

 high science. But in regard to this, it may be observed that 

 the present system of light-house apparatus, now in use in 

 every part of the civilized world, was invented and intro- 

 duced into practice in its minutest details by a man of ab- 

 stract science, the celebrated Fresnel, who shared with Young, 

 of England, the invention of the undulatory theory of light, 

 and its application to all the phenomena of optics. 



The light apparatus introduced by the Board as a substi- 

 tute for that previously in use is principally that of the French 

 system. But the Board has from the first been alive to the 

 introduction of improvements and has carefully considered 

 every suggestion and tested every invention which gave 

 promise of greater economy or efficiency. Instead of sperm - 

 oil, which was first employed, it has introduced, at one- 

 third of the cost, lard-oil, and with this, a required modifica- 

 tion of the lamps, particularly those of the larger kind, in 

 order that the oil may be burned at a higher temperature, 

 especially in the northern portions of the United States. 



But the greatest improvement which has been introduced 

 is that relative to fog-signals, — indispensable aids to naviga- 

 tion, especially on the northeastern, and western portions of 



