ioo POPULAR LECTURES AND ADDRESSES. 



piece of chalk is the fourth power of seven or 

 eight yards per hour ; yet it would be perfectly 

 good sense. 



Think now of an infinitesimal satellite revolving 



round the earth you ask, What is an infinitesimal 



satellite ? To be " infinitesimal " for our present 



purpose, it must be very small in comparison with 



the earth, so as not to cause sensible motion by 



its reaction on the earth. Well, a 5OO-lb. shot 



is an infinitesimal satellite; though it is not, 



perhaps, infinitesimal in some of its aspects. 



There must be no resistance of the air, of course. 



Now fire it off with such a velocity that it will 



have a very flat trajectory, neither more nor less 



flat than the earth, and it will continue going 



round and round the earth. Find the velocity at 



which you must fire off the shot to make it go 



round the earth, and, if there is no resistance of 



the air, there is our infinitesimal satellite. These 



somewhat pedantic words are justified, because 



" infinitesimal satellite " is nine syllables to 



express three or four sentences ; that is our 



justification. 



