428 THE PRINCIPLES OF SCIENCE. 



Determination of the Zero point by the Method 

 of Means. 



There are a number of important observations in which 

 one of the chief difficulties consists iu defining exactly the 

 zero point from which we are to measure. We can point 

 a telescope with great precision to a star and can measure 

 the angle through which the telescope is raised or lowered 

 to a second of arc ; but all this precision will be useless 

 unless we can know exactly where the centre point of 

 the heavens is from which we measure, or, what comes to 

 the same thing, the horizontal line 90 distant from it. 

 Since the true horizon has reference to the figure of the 

 earth at the place of observation, we can only determine 

 it by the direction of gravity, as marked either by the 

 plumb-line or the surface of a liquid. The question re- 

 solves itself then into the most accurate mode of observing 

 the direction of gravity, and as the plumb-line has long 

 been found hopelessly inaccurate, astronomers generally 

 employ the surface of mercury in repose as the criterion 

 of horizontality. They ingeniously observe the direction 

 of the surface by making a star the index. From the 

 Laws of Reflection it follows that the angle between the 

 direct ray from a star and that reflected from a surface 

 of mercury will be exactly double the angle between the 

 surface and the direct ray from the star. Hence the 

 horizontal or zero point is the mean between the apparent 

 place of any star or other very distant object and its 

 reflection in mercury. 



A plumb-line is perpendicular, or a liquid surface is hori- 

 zontal only in an approximate sense ; for any irregularity 

 of the surface of the earth, a mountain, or even a house 

 must cause some deviation by its attracting power. To 

 detect such deviation might seem very difficult, because 

 every other plumb-line or liquid surface would be equally 



