44 Domestic Science 



state of our knowledge. The " quantity of matter " 

 contained in any given body is termed its " mass " 

 and the unalterability of this mass is usually expressed 

 by the phrase " the mass of a body is constant ". 



25. We all know that a body, left unsupported in 

 the air, moves rapidly towards the Earth's surface and 

 finally comes to rest on that surface. It is probably 

 not so generally known that, while the body is travel- 

 ling towards the Earth, the Earth is at the same time 

 moving towards the body. This is actually the case, 

 the rates at which the Earth and the body move being 

 inversely proportional to their masses, whence it 

 results that, the Earth's mass being so much greater 

 than that of the body, the former's rate of movement 

 is so insignificant that we cannot realise it. 



Similarly any two bodies would move toward one 

 another if causes preventing this motion did not inter- 

 vene. We call " that which produces (or tends to 

 produce) motion in a body ", a force, and the mutual 

 tendency of all bodies to move towards one another 

 is said to be produced by the force termed " gravita- 

 tion " or, more briefly, by the " force of gravity ". 

 To the famous philosopher, Sir Isaac Newton, we owe 

 the discovery that the force of gravity is universal, 

 and it was the application of his discovery to astro- 

 nomical problems that first gave scientists a clear insight 

 into the means by which the heavenly bodies are made 

 to preserve their relative positions with respect to one 

 another. 



The universality of gravity must be clearly grasped. 

 If two objects say two footballs having no initial 

 motion were subject to no force save their own mutual 

 gravitational attraction, they would move towards one 

 another till they touched, and would remain in contact. 



