LECTURES. 193 



Terrestrial bodies are divided into tliree kingdoms — Animal, Vege- 

 table, and Mineral. Also into Organic and Inorganic. 



EXTENSION. 



(6.) Matter exists in space ; bodies occupy definite portions of space 

 and are therefore extended. 



Extension of matter in tliree dimensions. 



The quantity of space occupied by a body is called its volume. 



(7.) The exact dimensions of bodies are often required in scientific 

 investigations, and to obtain these, various instruments and methods 

 are employed. 



The vernier (named from the inventor.) Improperly called nonius. 

 In this, small divisions are measured by the difference of larger ones, 

 on two scales. The long graduated rod is called the scale, the short 

 slider the vernier. See model. 



As an example let each division on the scale be yV of ^^^ inch, and 

 let 11 of these be equal to ""O of those, of the vernier ; then it follows 

 that each division of the latter will be \]j of an inch, and the difference 

 of the two will be yio- of an inch. Now if any two divisions on each 

 scale coincide with each other, the next pair above will differ -^'^q of 

 an inch, the pair two degrees above -^1-^, three degrees yf y-; *^^- 



(8.) Comparator. For comparing lengths of bars. 



Micrometer, consisting of a fine screw with a large circular head 

 divided into parts — applied to different instruments. 



1. To dividing machine. Examples of use — 500 divisions in the 

 length of an inch, on a slip of glass. 



2. To spherometer — tripod with micrometer screw in the middle to 

 determine thickness and sphericity. 



Gage plate — for ascertaining the thickness of wire and of plates. 



Proportional callipers and compasses. 



Saxton's moving mirror, for measuring minute changes in length. 



(9.) Method of determining interior diameter of fine tubes, by 

 weighing the mercury which it will contain. 



Use of a vessel with a bulb and fine hollow stem, as in the ther- 

 mometer. 



Method of graduating irregular vessels into divisions of equal capa- 

 city by equal weights of mercury. 



IMPENETRABILITY. 



(10.) The property in consequence of which no two bodies can occupy 

 the same space at the same time. 



Sometimes regarded as an axiom, but rests on invariable experience. 



It was not recognized before the time of Archimedes, who made it 

 the ground of his theory of Hydrostatics. 



Space without impenetrability is called void space or a vacuum. 



Absolute impenetrability not considered by some as an essential 

 property of matter. See Theory of Boscovich. 



Illustrations — impenetrability of air ; water ; solids. 



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