Chase.] 4bO [juae 17, 



lines of investigation are guided by an understanding of tlie influences and 

 consequences of elastic action and reaction. 



66. Mass, Distance and Time. 



Laplace regarded force as proportional to velocity, treating momsntum, 

 or the product of mass by velocity, as the result of force acting on mass . 

 He defined velocity as "the ratio of the space to the time emploj^ed in de- 

 scribing it." More recent writers consider force as equivalent to vis viva, 

 involving both matter and motion, and represented by one-half the pro- 



duct of mass by the square of velocity, ~^ . Often, however, they fol- 

 low Laplace in treating velocity as a ratio, designating distance or length 

 by I, and time by t. The symbols m, I, t, therefore, are applicable in all 

 discussions of force, and their use has paved the way for a ready accep- 

 tance of any evidence which may be adduced respecting the substantial 

 unity of force. The fluctuating and arbitary values which have been 

 assigned to the symbols, and the lack of any universally recognized stand- 

 ards of measurement, have obstructed the discovery of evidence without 

 weakening the belief in kinetic unity. Even the recognition of a com- 

 mon velocity, in light and electricity, seems to have led to no systematic 

 search for common standards of length and time. 



67. Maxima, Minima and Means. 



In all ordinary investigations, in any special department of dynamics, 

 we deal with observed values which have a limited range, with a view of 

 finding the most probable mean. In the study of forces which vary in 

 accordance with known or supposed law, minimum or vanishing values 

 claim special consideration. For a full understanding of any force Ave 

 must also know the greatest energy of which it is capable, or of which we 

 can obtain any practical experience. If we wish to compare different kinds 

 of force, in order to ascertain the character of supreme and controlling 

 energy, it seems especially important that we should turn our attention, 

 at the outset, to the greatest manifestations of each of the elements of 

 energy, m, I, t. 



68. Light and Electricity. 



For a long time, little was known of electricity except the phenomena of 

 attraction, repulsion, and luminous manifestation. Faraday's want of 

 success in searching for some bond of union between electricity and grav- 

 itation, deterred others from continuing the search. The undulatory 

 theory of light enabled Weber and Kohlrausch to make their first deter- 

 mination of a velocity, which serves to connect many electrostatic and 

 electromagnetic phenomena, and which is of the same order of magni- 

 tude as the velocity of light, v^ . Their subsequent investigations, together 

 with those of Thomson, Maxwell, Ayrton and Perry, showed that the two 

 velocities are not only of the same order of magnitude, but that they are 



