THE CONQUEST OF THE ZONES 



earth's surface, the difference between which, as 

 measured in degrees and minutes, was accurately known. 



In June of the year 1633, the Englishman Robert 

 Norman made very accurate observations of the alti- 

 tude of the sun on the day of the summer solstice (when 

 of course it is at its highest point in the heavens); the 

 observation being made with a quadrant several feet in 

 diameter stationed at a point near the Tower of London. 

 On the corresponding day of the following year he 

 made similar observations at a point something like 

 1 25 miles south of London, in Surrey. The two observa- 

 tions determined the exact difference in latitude be- 

 tween the two points in question. 



Norman then undertook a laborious survey, that he 

 might accurately measure the precise distance in miles 

 and fractions thereof that corresponded to these known 

 degrees of latitude. He made actual measurements 

 with the chain for the most part, but in a few places 

 where the topography offered peculiar difficulties he 

 was obliged to depend upon the primitive method of 

 pacing. 



The modem surveyor, equipped with instruments 

 for the accurate measuring of angles, not differing 

 largely in principle from the quadrant of the naviga- 

 tor, would consider Norman's method of measurement 

 a very clumsy one. He would measure only a single 

 original base line of any convenient length, but would 

 make that measurement with very great accuracy, 

 using, perhaps, a rod packed in ice that it might not 

 vary in length by even the fraction of an inch through 

 changes in temperature. An accurate base line thus 



[35] 



