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case appeared to be coincident with the variations of an aneroid barometer or a thermometer, but in every instance it was affected by the direction of the wind. The variation in the distinctness of the sound of a distant instrument as depending on the direction of the wind is so marked that we are warranted in considering it the principal cause of the inefficiency in certain cases of the most powerful fog-signals.

"In the remainder of his paper, as read in the presence of Prof. Tyndall, the chairman of the Lighthouse Board applied the hypothesis of Prof. Stokes to an explanation of certain abnormal phenomena of sound which had been observed during the course of his systematic inquiries with regard to the causes which affect the intensity of sound."

The reader now has the whole case before him. This is the substance of what Prof. Tyndall listened to in Washington, and for not recognizing which, to the credit of American science, in his book on sound, he has been the subject of a bitter and persistent newspaper attack. Prof. Tyndall says that the reading of the document left him in mental perplexity, and we are certainly not surprised at his state of mind. The subject, it is to be remembered, was not new to him. He had been for years engaged in the scientific service of the English Lighthouse Department; he had been an explorer in the field of acoustics, and was familiar with the history of the subject. He knew that it was involved in obscurity, that observations disagreed, and that there was much theoretical conflict about it. Nothing seemed established, and he states that Prof. Henry's paper left him still in an intellectual fog in regard to the whole question. The reader will see that the statement is pervaded by doubt. Conflicting opinions are given, and the prominent question was yet to be decided by the aid of Boston pilots. Finally, a conjecture, thrown out by an English physicist, is invoked for the explanation of anomalous effects observed. Clearly it was a case for further and formidable work which required to be met by a comprehensive, systematic, and thorough-going research. Prof. Henry's paper settled nothing. That it was without value as a contribution to science, we by no means assert; but every one can see that it was not the product of a full, methodical, and exhaustive inquiry, such as the subject urgently demanded and had not yet received from any source. The observations of Humboldt, early in the century, on the passage of sound, were important, as Prof. Tyndall himself attests, but to characterize them as a "systematic inquiry into the causes which affect the intensity of sound in the atmosphere" is simply absurd. Humboldt confined himself to one branch of the investigation, and whole tracts of it he did not touch.

Prof. Tyndall was, therefore, abundantly justified in assuming that the blank of 167 years had not been filled up; and, being deeply interested in the subject, and having command of the means for an elaborate course of researches upon it, he determined to enter fully into the inquiry, with the hope of dispelling some of the uncertainty which clouded it.

He took up the question from a purely scientific point of view, not to improve the art of fog-signaling or arrive at any immediate practical results valuable to the navigator, but simply to test theories, explain phenomena, harmonize discrepancies, and advance acoustical science. He attacked the problem of the "causes" which affect the intensity of sound in the air with a single-mindedness, a rigor of method, and a completeness of resources, that had never before been employed upon it. His researches went on in a double series, on the coast and in the laboratory. Using the facilities furnished by the Government at home, and sending abroad for the best that could be supplied, he carried on his observations and experiments on a large scale from the South Foreland Station, scrutinizing