RESULTS OF BRITISH TRANSIT EXPEDITIONS. 67 



British operations alone should not seem to be altogether 

 satisfactory. For it may still happen that that portion of 

 the British operations which only has value when combined 

 with the work of other countries may be found to possess 

 extreme value. We had good reason for doubting before- 

 hand whether results of any great value could be obtained 

 by Delisle's method. It was only because Halley's was sup- 

 posed to fail totally that the Government astronomers ever 

 thought of employing that method, which the experience of 

 former transits had taught us to regard as of very little value. 



It may be asked, however, how we are to form an 

 opinion from the result of calculations based on the 

 Delislean operations during the last transit, whether the 

 method is satisfactory or not. If as yet the sun's distance 

 is not exactly determined, a result differing from former 

 results may be better than any of them, many will think ; 

 and therefore the method employed to obtain it may be 

 more satisfactory than others. If, they may reason, we place 

 reliance on a certain method to measure for us a certain 

 unknown distance, how can we possibly tell from the distance 

 so determined whether the method is trustworthy or not? 



Perhaps the readiest way of removing this difficulty, and 

 also of illustrating generally the principles on which the 

 determination of the most probable mean value of many 

 different estimates depends, is by considering a familiar 

 experience of many, a case in which the point to be deter- 

 mined is the most probable time of day. Suppose that we 

 are walking along a route where there are several clocks, 

 the time shown by our own watch being, for whatever 

 reason, open to question. We find, say, that as compared 

 with our watch time, one clock is two minutes fast, the 

 next three minutes fast, the next one minute slow, and 

 so on, two or three perhaps being as much as six or seven 

 minutes fast, and two or three being as much as three or four 

 minutes slow as compared with the watch. We note, how- 

 ever, that these wider ranges of difference occur only in 

 the case of clocks presumably inferior cheap clocks in small 



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