I So Mimttenefs and Rarity of Light. [Book III. 



light moves about two million of times as faft as a 

 cannon ball *. The force with which moving bodies 

 ftrike, is in proportion to their mafles multiplied by 

 their velocities ; and confequently if the particles of 

 light were equal in bulk to the two millioneth part 

 of a grain of fand, we mould be no more able to en- 

 dure their impulfe than that of fand fliot point blank 

 from the mouth of a cannon f. The minutenefs of 

 the rays' of light is alfo demonftrable from the facility 

 with which they penetrate glafs, cryftal, and other 

 folid bodies, which have their pores in a rectilinear 

 direction, and that without the fmalleft diminution of 

 their velocity, as well as from the circumftance of 

 their not being able to remove the fmalleft particle 

 of microfcopic duft or matter which they encounter 

 in their progrefs. A further proof might be added, 

 jhat if a candle is lighted, and there is no obftacle to 

 obftruct its rays, it will fill the whole fpace within two 

 miles around it alrroft inftantaneoufly, and before it 

 has loft the lead fenfible part of its fubftance J. 



To the velocity with which the particles of light 

 are known to move may, in a great meafure, be at- 

 tributed the extreme rarity an.d tenuity of that fluid. 

 It is a well-known fact, that the effect of light upon 

 the eye is not inftantaneous, but continues for a con- 

 iiderable time . Now we can fcarcely conceive, a 

 more minute divifion of time than the one hundred 

 and fiftieth part of a fecond. If, therefore, one lucid 



* A cannon ball flies with the velocity of about a mile in eight 

 feconds. ' Nichol foil's Phil. vol. i. p. 257. During the late fiege 

 of Gibraltar, there were two boys, who ufed to be Rationed on the 

 works, and whofe quick fight enabled them to gi\ r e notice to the 

 workmen of the approach of a ball from the enemy's works. 

 *~*DttnfawattF > t Gib, 



f Nicholfon's Phil. vol. i. p. 257. J Enficld's Phil. 131, 



Nicholfon's Phil. vol. i, p. 25^. 



point 



