RESULTS OF BRITISH TRANSIT EXPEDITIONS. 65 



the exact Greenwich time (or any other selected standard 

 time) at his station, or in other words he must know exactly 

 how far east or west his station is from Greenwich (or some 

 other standard observatory). For all the observations made 

 by this method must be compared together by some absolute 

 time standard. In the Halleyan method the duration of 

 transit only is wanted, and this can be as readily determined 

 by a clock showing local time (or indeed by a clock set 

 going a few minutes before transit began and showing wrong 

 time altogether, so only that it goes at the right rate) as by 

 a clock showing Greenwich, Paris, or Washington time. The 

 clock must not gain or lose in the interval. But a clock which 

 would gain or lose appreciably in four or five hours, would be 

 worthless to the astronomer ; and any clock employed for 

 scientific observation might safely be trusted for an interval of 

 that length ; whereas a clock which could be trusted to retain 

 true time for several days, is not so readily to be obtained. 

 We need not consider here the origin of the misappre- 

 hension (under which our principal Government astronomer 

 lay for some time), that the Delislean method was alone 

 available during the transit of 1874, the Halleyan method, 

 to use his words, l failing totally.' The British stations 

 were selected while this misapprehension remained as yet 

 uncorrected. Fortunately the southern stations were suit- 

 able for both methods. The northern were not : for this 

 reason, simply, that one set were so situated that night 

 began soon after the beginning of transit, which alone 

 could be observed; while the other set were so situated 

 that night only came to an end a short time before the 

 transit ended, so that the end of transit only could be 

 observed. No doubt when the mistake just mentioned 

 had been clearly recognised, as it was early in 1873, 

 measures would have been taken to rectify its effect by 

 occupying some suitable northern stations for observing 

 the whole transit, if Great Britain had been the only nation 

 taking part in the work. Fortunately, however, other nations 

 might be trusted to occupy the northern region, which had 



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