344 SCIENTIFIC THOUGHT. 



24. of the last century, subjected the less universal terrestrial 



Electrical 



and mag- phenomena or magnetic and electric action to exact mea- 



netic action. 



surements, finding that a formula corresponding to the 

 gravitation formula described them with surprising ac- 

 curacy, with this remarkable difference, that here not only 

 attractive but also repulsive forces, following the same 

 mathematical relations as to mass and distance, came into 

 play. To these confirmatory discoveries must be added 

 the measurement of the intensity of radiations which 

 proceed from centres, such as those of light and heat, 

 made by various philosophers during the latter half of 

 the last century. Newton, and his great successor La- 

 place more than a century after him, both favoured the 

 emission or emanation hypothesis of light, and it was 

 thus natural to fasten upon the analogy which existed 

 between the intensity in which radiation, gravitation, and 

 electric and magnetic action change with the distance 



25. from their respective centres. All these agencies came 

 emanations, thus under the general conception of forces emanating 



from fixed centres, and spreading through space, in the 

 proportion of the superficial area of the spheres described 

 around their centres with increasing radii i.e., decreasing 

 or becoming diluted in the ratio of the squares of these 

 radii or distances. These analogies were indeed recognised 

 to be very imperfect, inasmuch as light and radiant heat 

 occupy a measurable time to spread from their centres, 

 whereas the time occupied by the force of gravitation is 



especially Miething, ' L. Euler's 

 Lehre vom Aether,' Berlin, 1894. 

 In the course of this century the 

 mechanical theory of gravitation, 

 including the attempts of Lesage, 

 Euler, Huygens, and Newton him- 



self, has again received attention 

 through Faraday's, Maxwell's, and 

 Hertz's electric theories, and Win. 

 Thomson (Lord Kelvin) has especi- 

 ally studied the ideas of Lesage. 

 Of this more later on. 



