224 A Short History of Astronomy [Cn. ix. 



passed over here, but certain results of special astronomical 

 importance require to be mentioned. 



Galilei, as we have seen (chapter vi., 130, 133), 

 was the first to enunciate the law that a body when once 

 in motion continues to move in the same direction and 

 at the same speed unless some cause is at work to make 

 it change its motion. This law is given by Newton in 

 the form already quoted in 130, as the first of three 

 fundamental laws, and is now commonly known as the 

 First Law of Motion. 



Galilei also discovered that a falling body moves with 

 continually changing velocity, but with a uniform accelera- 

 tion (chapter vi., 133), and that this acceleration is the 

 same for all bodies (chapter vi., 116). The tendency of 

 a body to fall having been generally recognised as due 

 to the earth, Galilei's discovery involved the recognition 

 that one effect of one body on another may be an accelera- 

 tion produced in its motion. Newton extended this idea 

 by shewing that the earth produced an acceleration in the 

 motion of the moon, and the sun in the motion of the 

 planets, and was led to the general idea of acceleration in 

 a body's motion, which might be due in a variety of ways 

 to the action of other bodies, and which could conveniently 

 be taken as a measure of the effect produced by one body 

 on another. 



1 80. To these ideas Newton added the very important 

 and difficult conception of mass. 



If we are comparing two different bodies of the same 

 material but of different sizes, we are accustomed to think 

 of the larger one as heavier than the other. In the same 

 way we readily think of a ball of lead as being heavier 

 than a ball of wood of the same size. The most prominent 

 idea connected with " heaviness " and " lightness " is that 

 of the muscular effort required to support or to lift the 

 body in question ; a greater effort, for example, is required 

 to hold the leaden ball than the wooden one. Again, the 

 leaden ball if supported by an elastic string stretches it 

 farther than does the wooden ball ; or again, if they are 

 placed m the scales of a balance, the lead sinks and the 

 wood rises. All these effects we attribute to the " weight " 

 of the two bodies, and the weight we are mostly accustomed 



