1877] WRITINGS OF JOSEPH HENRY. 519 



These faculties of the mind may all be much improved 

 and strengthened by practice. The most important requisite 

 however to scientific investigations of this character, is a mind 

 well stored with clear conceptions of scientific generalizations, 

 and possessed of sagacity in tracing analogies and devising 

 hypotheses. 



Without the use of hypotheses or antecedent probabilities, 

 as a general rule — no extended series of investigations can 

 be made as to the approximate cause of casual phenomena. 

 They require to be used however with great care, lest they 

 become false guides which lead to error rather than to truth. 



It is not enough for a physical investigation that we have 

 the simple idea, which may be embodied in a mathematical 

 equation ; we must see clearly with the mind's eye the oper- 

 ations in nature, and how the phenomena are produced in 

 accordance with the well-known laws of force and motion. 



An Investigation of an Acoustic Phenomenon. — As an illus- 

 tration of what I have said, as well as an original scientific 

 communication, I may be allowed to present in this connec- 

 tion an account of some observations on the phenomena of 

 sound in its application to fog-signals, in which I have been 

 engaged during the past summer, and which are an exten- 

 sion of the investigation of whose progress I have given an 

 account at different times to the society. 



This year my attention was again directed to the peculiar 

 effect observed for several years past on the coast of Maine, 

 which has been classed among those to which the term " ab- 

 normal phenomena of sound" is applied. In August, 1873, 

 this was partially examined, and the result published in the 

 Light-House Report for 1874. In order to investigate it fur- 

 ther, I associated myself with General J. C. Duane, engineer 

 of the first Light-House district; Commander H. F. Picking, 

 inspector of the same district; Mr. Edward L. Woodruff", 

 assistant engineer of the third district, and Mr. Charles 

 Edwards, assistant engineer of the first district. 



The phenomenon to be investigated was exhibited in con- 

 nection with the fog-signal at a station called Whitehead, on 



