478 THE POPULAR SCIENCE MONTHLY, 



The reader who turns to this seventh chapter will find that it 

 opens with an " introduction " professing to give " a summary of 

 existing knowledge " in the matter of fog-signaling. The writer 

 states that while the velocity of sound has formed the subject of re- 

 peated and refined experiments by the ablest philosophers, " the pub- 

 lication of Dr. Derham's celebrated paper in the ' Philosophical Trans- 

 actions ' for 1708 marks the latest systematic inquiry into the causes 

 which affect the hxtensity of sound in the atmosphere.'''' And, after 

 making this statement, the professor immediately adds as follows : 

 " Jointly with the Elder Brethren of the Trinity House, and as their 

 scientific adviser, I have recently had the honor of conducting an in- 

 quiry designed to fill the blank here indicated.'''' In order still further 

 to impress on the reader a sense of the magnitude of this " blank," 

 Dr. Tyndall indulges in one or two preliminary references which, he 

 says, " will sufiice to show the state of the question when this [his] 

 investigation began." The first of these references cites the opinion 

 of Sir John Herschel to the efiect that fogs and falling rain, and more 

 especially snow, had been found by Derham " to tend powerfully to 

 obstruct the propagation of sound." The second of his references is 

 made to what he calls "a very clear and able letter " addressed by 

 Dr. Robinson, of Armagh, to the British Board of Trade in 1863. In 

 this " very clear and able letter " Dr. Robinson states that sound is 

 the only known means for coping with fogs, but about it, he adds, 

 " the testimonies are conflicting, and there is scarcely one fact relating 

 to its use as a signal lohich can he considered as established.'''' But 

 Dr. Robinson is clear on one point to wit, that " fog is a powerful 

 damper of sound." 



On the strength of these historical references. Dr. Tyndall ven- 

 tures the remark that, prior to the investigation conducted by him, 

 the views enunciated i;nder this head by Derham, Herschel, and Rob- 

 inson, " wei'e those universally entertained." It was in order to fill 

 " the blank " indicated by the universal prevalence of such erroneous 

 opinions that his inquiry, he says, was set on foot. And his inquiry, 

 he tells us, was begun May 19, 1873. 



Now, it is a matter, not only of scientific knowledge, but of pub- 

 lic notoriety in this country, that extensive researches on " the causes 

 which afiect the intensity of sound in the atmosphere " had been 

 made by the United States Lighthouse Board long before Prof. 

 Tyndall began his investigations. That he should have chosen to 

 ignore the fact in the body of his present volume becomes only the 

 more surprising when, on turning to its preface, we find that be was, 

 as he confesses, " quite aware in a general way that labors, like those 

 now for the first time made public, had been conducted in the United 

 States," and " this knowledge," he subjoins, " was not without influ- 

 ence upon my conduct." If his knowledge of the similar labcj's con- 

 ducted under this head in the United States was not, as he acknowl- 



