166 BULLETIN OP THE 



is conscious of the pleasure of the self-appreciation which flows 

 from having been initiated into the secrets of nature, and allowed 

 the place not merely of an humble worshipper in the vestibule of 

 the temple of science, but an officiating priest at the altar. 



In this sketch which I have given of a successful investigation, 

 it will be observed that several faculties of the mind are called 

 into operation. First, the imagination, which calls forth the forms 

 of things unseen and gives them a local habitation, must be active 

 in presenting to the mind's eye a definite conception of the modes 

 of operation of the forces in nature sufficient to produce the phe- 

 nomena in question. Second, the logical power must be trained 

 in order to deduce from the assumed premises the conclusions 

 necessary to test the truth of the assumption in the form of an 

 experiment, and again the ingenuity must be taxed to invent the 

 experiment or to bring about the arrangement of apparatus adapt- 

 ed to test the conclusions. 



These faculties of the mind may all be much improved and 

 strengthened by practice. The most important requisite, how- 

 ever, to scientific investigations of tliis character, is a mind well 

 stored with clear conceptions of scientific generalizations and 

 possessed of sagacity in tracing analogies and devising hypo- 

 theses. 



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 operations in 

 nature, and how the phenomena are produced in accordance with 

 the well-known laws of force and motion. 



As an illustration of what I have said, as well as an original 

 scientific communication, I may be allov/ed to present in this con- 

 nection an account of some observations in which I have been 

 engaged during the past summer; and which are an extension of 

 the investigation of the phenomena of sound in its application to 

 fog-signals, of the progress of which I have given an account at 

 different times to the Society. 



This year, my attention was called to a special phenomenon 



