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 repeated and refined experiments by the ablest philosophers, "the publication of Dr. Derham's celebrated paper in the 'Philosophical Transactions' for 1708 marks the latest systematic inquiry into the causes which affect the intensity 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 inquiry 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 suffice 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 effect 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 which 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On the strength of these historical references, Dr.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 ventures the remark that, prior to the investigation conducted by him, the views enunciated under this head by Derham, Herschel, and Robinson, "were 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 public notoriety in this country, that extensive researches on "the causes which affect 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 he 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 influence upon my conduct." If his knowledge of the similar labors conducted under this head in the United States was not, as he acknowl-
