THE COMING TRANSIT OF VENUS. 17 



subject does not labour under the fault of being prema- 

 ture. On the contrary, the time is now at hand when 

 a final decision must be made as to the course which 

 this country is to pursue ; and inasmuch as my purpose 

 is not solely to describe what is being done, but to 

 point out what (in my opinion) should be done, the 

 present is the proper time to speak. 



A surveyor, who wishes to determine the distance of 

 an inaccessible object, measures a convenient base-line 

 and observes the direction of the object as seen from 

 either end of the line. He thus has the base and 

 the two base-angles of a triangle ; and the simplest 

 geometrical considerations teach that the other two 

 sides of the triangle can thence be determined. These 

 sides are, of course, the distances of the inaccessible 

 object from the two ends of the base-line. Now this is 

 the fundamental method employed by astronomers to 

 determine the distances of the celestial bodies. It is 

 applied directly to the moon. An observer at Green- 

 wich (let us say), notes the direction of the moon when 

 at her highest, or due south ; another at Cape Town 

 (let us say), does the like ; then a line joining Green- 

 wich and Cape Town is a base-line of known length, 

 and the two directions give the base-angles. The 

 triangle is a very long one, its vertical angle (that is, 

 the angle opposite the base) being one of about a degree 

 and a half, or about the angle swept out by the hand 

 of a clock or watch during a quarter of a minute ; but 

 such a triangle is quite within the methods of treat- 

 ment available to astronomers. 



c 



