LA"WS OF TALLIN; G BODIES. 



53 



SECTION IV. 



EFFECTS OF GRAVITY AS DISPLAYED BY FALLIN'O BODIEa 



What is a Ver- 

 tical Line ? 



Wliat is a 

 Plumb Line? 



Fig. 25. 



98. Wiien an unsupported body falls, its 

 motion will be in a straight line toward the 



center of the earth. This line is called a Vertical 



Line. 



99. If a body be suspended by a thread, the 

 thread will always assume a vertical direction, 



or it will represent that path in which the body would 

 have fallen. A weight thus suspended by 

 a thread, is called a Plumb-Line,* Fig. 25, 

 and is used by carpenters, masons, etc., to 

 ascertain by comparison, whether their work 

 stands in a vertical or perpendicular position. 

 What is a 100. A plumb-line is always 



Level Surface? perpcudicular to the surface of 

 water at rest. The position of such a sur- 

 face we call Level. 



No two plumb-lines upon the earth's surface will be 

 parallel, but will incline toward each other, since no two 

 bodies from different points can approach the center of a 

 sphere in a parallel direction. If their distance apart be 



one mile, this inclination will amount to one minute, 



and if it be sixty miles, to one degree. In Fig. 26, 



let E E be a portion of the earth's surface, and D its 



center; the bodies A, B, and C, when allowed to 



drop, will fall in the direction A D, B D, and C D. 



101. As the attraction of e- 



the earth acts equally and 



independently on all the 



particles composing a body, 

 it is clear that they must all fall with 

 equal velocities. It makes no difference whether the sev- 

 eral particles fall singly, or whether they fall compacted 

 together, in the form of a large or a small body. 



• Plumb Line, so called from the Latin word plumbum, lead, the weight usually at- 

 tached to the string. 



aA 



Will all bodies, 

 under the in- 

 fliienccofgi'av- 

 ity alone, fall 

 with equal ve- 

 locities ? 



