JOSEPH HENRY. 365 



whistle, trumpet, and siren for fog signals, deciding in favour 

 of the last named. This problem, however, had many perplex- 

 ing sides, and occupied a large share of his attention for the 

 last twelve years of his life. 



" The value of Henry's services to the various executive 

 departments of our Government, faithfully and unostenta- 

 tiously performed through a long series of years and a suc- 

 cession of presidential administrations, can not be estimated," 

 says W. B. Taylor, in the admirable discourse prepared for 

 the Philosophical Society of Washington, from which much of 

 the material of the present account has been taken. " When- 

 ever in any important case a scientific adviser could be useful 

 to the proper conduct of a bureau, Henry's reputation gener- 

 ally pointed him out as the most suitable expert and arbiter. 

 On the outbreak of the great Civil War, the number of such 

 references was naturally very considerably increased. The 

 departments of War, of the Navy, and of the Treasury were be- 

 sieged by projectors with every imaginable and impossible 

 scheme for saving the country and demolishing the enemy. 

 Torpedo balloons, electric-light balloons, wonderful com- 

 pounds destined to supersede gunpowder and revolutionize 

 the art of war ; cheap methods for the manufacture of Govern- 

 ment bonds and paper money ; multitudinous expedients for 

 the prevention of counterfeiting by devices in the engraving, 

 by secret markings, by anti-photographic inks, by peculiar tex- 

 tures of paper (applicable to coupons, to circulating notes, 

 to revenue stamps), each warranted to be infallible such 

 were among the agencies by which patriotic patentees and 

 adroit adventurers were willing to serve their country, and to 

 reap their reward by the moderate royalty or percentage due 

 to the magnificence of the public benefit." When it was pro- 

 posed that the pay of an expert should be given for these serv- 

 ices Henry refused, preferring to make them a free gift to the 

 country. 



With so many exacting duties is it to be supposed that 

 Henry would find any opportunity to use his splendid talents 

 for original research ? He had no expectation of it when he 

 left Princeton, yet in spite of the improbability he made some 

 valuable advances in directions of his own choice. He so im- 

 proved the thermo-galvanic multiplicator of Nobili and Melloni 

 as to produce his wonderfully sensitive "thermal telescope" 



