200 LECTURES. 



Illustration. Attraction at a distance — action not interrupted by 

 solid matter — attraction and repulsion through the human body. 

 Attraction and repulsion instantaneous. 



Variation of intensity with change of distances. 



Experiment to show phenomena which appear the result of attrac- 

 tion, but which are due to pressure, &c. 



pieces of wood collected together on water not the result of direct 

 attraction. 



Attraction of Gravitation. 



(46.) The reciprocal tendency of all parts of the solar system to 

 approach each other. 



The same action probably extends to other systems. 



(47.) Gravitation is an incessant force, and is generally measured 

 hy the velocity tvhich it imparts to the attracted body in a second of time. 

 May also be measured by pressure. Illustrations. 



Newton's Theory of Universal Gravitation. The most extended 

 generalization ever established by man. It may be expressed as fol- 

 lows : 



1. The attraction exists hetiveen the atoms of all matter at finite dis- 

 tances, and is the same for all kinds of matter, hence: 



2. The force of attraction is proportional to the mass of the attracting 

 body; the distance being the same. 



3. If the same body attracts several bodies at different distances, the 

 foi'ces are inversely as the square of the distances. 



All deductions from this theory are in strict accordance with the 

 phenomena of nature. The only proof of the truth of any physical 

 law. 



(48.) In some cases of attraction the whole moving fwce of approach 

 of two bodies is required, and this is as the product of the masses into 

 the inverse square of the distance. 



The acceleration of the velocity of approach is as the sum of the two 

 masses, and inversely as the square of the distance. 



Illustrations of the laws by diagrams of atoms. 



(49.) In reference to the attraction of spheres the following propo- 

 sitions will be proved. See Mechanics. 



1. A particle of matter placed without a solid homogeneous sphere 

 is attracted as if all the matter of the si^here were in its centre. 



2. The attraction is the same in reference to a particle without a 

 hoUoiv sphere. 



3. A particle placed within a homogeneous hollow sphere is in 

 equilibr.^ at any point within the sphere. 



4. Particles placed at different distances from the centre within the 

 surface of a solid homogeneous sphere are attracted towards the cen- 

 tre with forces proportional to the distances from the centre. 



(50.) Attraction of spheroids. Gravitation the most feeble of all 

 attractions ; almost imperceptible between small masses ; long time 

 required for two lead balls to come together. 



Illustrations of the foregoing principles. 



