THE CONQUEST OF TIME AND SPACE 



vague the deductions must have been from the ob- 

 servations of navigators, however accurately made, 

 prior to the time when such tables as those of the Nau- 

 tical Almanac had been prepared. Fully to appreciate 

 this, it is necessary to understand that the observations 

 supplied in such profusion for the use of the mathe- 

 matical astronomer are in themselves subject to errors 

 that might seriously vitiate the results of the final com- 

 putation. They must, therefore, be made with the ut- 

 most accuracy, and with instruments specially pre- 

 pared for the purpose. The chief of these instruments 

 is not the gigantic telescope but the small and rela- 

 tively simple apparatus known as a transit instrument. 

 This constitutes essentially a small telescope poised on 

 very carefully adjusted trunnions, in such a way that it 

 revolves in a vertical axis, bringing into view any celes- 

 tial body that is exactly on the meridian, and bodies in 

 this position only. To make observation of the transit 

 — that is to say the passage across the meridian line — 

 of any given body more accurate, the transit instru- 

 ment has stretched vertically across the center of its 

 field of vision a spider web, or a series of parallel spider 

 webs; in order, in the latter case, that the mean time 

 of several observations may be taken. 



So exceedingly difficult is it to manufacture and 

 mount an instrument of requisite nicety of adjustment, 

 that the effort has almost baffled the ingenuity of the 

 mechanic. Sir George Airy, in making a transit instru- 

 ment for use at the Royal Observatory at Greenwich, 

 required the trunnions on which it was to be mounted 

 to be ground truly cylindrical in form within a varia- 



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