ON ERRORS OP OBSERVATION. 14p 



a book on the practical part of the science, might imagine 

 that no part of the subject pretended even to ordinary 

 accuracy. Nothing appears to be done which is unaf- 

 fected by serious error ; and it seems as if a little more 

 attention to the fabrication of instruments would render 

 nine tenths of what has been written altogether useless. 

 This appearance is the victory of the head over the 

 hands ; the means of detecting the errors of instru- 

 ments are much more powerful than those of correcting 

 them. It is also the victory of astronomy over the 

 other physical sciences, on our knowledge of which the 

 manufacture of utensils depends : we know more of 

 the laws which regulate the changes of the heavens 

 than we do of those on which the stability and fluctu- 

 ation of instruments depend. Nor does the semblance 

 above mentioned entirely spring from unavoidable 

 error : for it is frequently the most convenient plan to 

 allow an error to subsist which might be corrected at 

 once, but which may be more easily corrected at 

 another stage of the process. It is also sometimes 

 useful to allow an error to remain of a larger magnitude 

 than is physically necessary, if by that means another 

 risk of error may be avoided. For instance, if re- 

 quisite correction be either an addition or a subtraction, 

 sometimes one and sometimes the other, the most 

 practised calculator will be very liable to confound the 

 two. This may be remedied by allowing to the instru- 

 ment a fixed error, either additive or subtractive, of 

 such a magnitude that casual fluctuations will never 

 alter its name. The correction, therefore, will always 

 require the same process, and the risk of error arising 

 from taking the wrong method will be avoided. 



2. The next plan of eliminating the fixed errors 

 of an instrument is by giving it such a construction 

 that an observation can be made in two different ways, 

 in which the fixed error must necessarily have dif- 

 ferent signs, and must be of the same amount in both 

 cases. This is in reality a method of making the 

 positive and negative errors of the same amount in 

 L 3 



