218 DISCOURSE ON THE STUDY 



skill. No better way of judging could be devised than 

 by letting him fire a hundred shots at it, and marking 

 where they all struck. Suppose this done, suppose 

 the wafer has been hit once or twice, that a certain 

 number of balls have hit the wall within an inch of 

 it, a certain number between one and two inches, 

 and so on, and that one or two have been some feet 

 wide of the mark. Still the question arises, what 

 estimate are we thence to form of his skill ? how 

 near (or nearer) may we, after this experience, 

 safely, or at least not unfairly, bet that he will come 

 to the mark the next subsequent shot? This the 

 laws of probability enable us on such data to say. 

 Again, suppose, before we were allowed to measure 

 the distances, the wafer were to have been taken 

 away, and we were called upon, on the mere evi- 

 dence of the marks on the wall, to say where it had 

 been placed ; it is clear that no reasoning would en- 

 able any one to say with certainty ; yet there is as- 

 suredly one place which we may fix on with greater 

 probability of being right than any other. Now, 

 this is a very similar case to that of an observer 

 an astronomer for example who would determine 

 the exact place of a heavenly body. He points 

 to it his telescope, and obtains a series of results 

 disagreeing among themselves, but yet all agreeing 

 within certain limits, and only a comparatively small 

 number of them deviating considerably from the 

 mean of all ; and from these he is called upon to 

 say, definitively, what he shall consider to have 

 1>een the most probable place of his star at the 

 moment. . Just so in the calculation of physical 

 data; where no two results agree exactly, and 



