ii Tlie Substance of Nature 19 



would have pointed in turn all round the room ; or if the 

 table were in the open air, all round the horizon. The space 

 of a whole turn is divided into 4 equal quadrants or quarters, 

 each of which is called a right angle, and the amount of 

 turn in a right angle is divided into 90 equal steps called 

 degrees (), each degree being the 36oth part of a whole 

 turn. Every degree is subdivided into 60 equal parts called 

 minutes ( ' ), and each minute into 60 parts called seconds 

 ("). An angle of i" is thus simply a short name for "the 

 i, 296,oooth part of a whole turn," and small though this 

 i s> _ij. of a second or less can be measured in fine instru- 

 ments. The amount of turning from the horizon or sky- 

 line of a plain to the zenith or point directly overhead is 

 one-quarter of a complete turn or a right angle, i.e. 90. 

 Degrees, minutes, and seconds are thus simply fractions of 

 the unit which is a turn ; and a turn is the same whether 

 the turning line sweeps round the horizon, the Earth's 

 equator, or a watch dial. 



32. Position by Angles. By fixing points from which 

 to begin the reckoning it is evident that two sets of 

 angles will enable one to define the position of any object 

 on a sphere, such as the sky. In the case of a star, for 

 example, by taking the north point of the horizon as a zero, 

 one first measures the number of degrees, minutes, and 

 seconds of turn until directly under the star, noting in 

 which direction (toward east or west) the turn is taken. 

 Then from the horizon at that point one measures the 

 number of degrees, minutes, and seconds of turn toward the 

 zenith to the star. Angular distance round the horizon is 

 called the azimuth of a point ; angular dis-tance toward the 

 zenith from the horizon is called its altitude. The instru- 

 ment which is most convenient and most generally used for 

 measuring angles of all kinds is the Sextant. 



33. Measurement of Distances by Angles. Every 

 one must have noticed in passing a church clock that if, 

 when standing directly opposite it, he sees the long hand 

 pointing exactly to XII, yet from a little distance on one 

 side he sees it to be a few minutes before, and from the 

 same distance on the other side a few minutes past the 



