104 THE PRINCIPLES OF SCIENCE. 



the sum of the approximate numbers 34*70, 52*693, 8o'i, 

 is 167-5 within less than '07. So far as I know Mr. 

 Sandeman is the only mathematician who has traced out 

 the rules of approximate arithmetic, and his directions are 

 worthy of careful attention c . Although the accuracy of 

 measurement has so much advanced since the time of 

 Leslie, it is not superfluous to repeat his protest against 

 the unfairness of affecting by a display of decimal frac- 

 tions a greater degree of accuracy than the nature of the 

 case requires and admits d . I have known a scientific 

 man to register the barometer to a second of time when 

 the nearest quarter of an hour would have been amply 

 sufficient. Chemists often publish results of analysis to 

 the ten-thousandth or even the millionth part of the 

 whole, when in all probability the processes employed can- 

 not be depended on beyond the hundredth part. It is 

 seldom desirable to give more than one place of figures of 

 uncertain amount ; but it must be aUowed that a nice per- 

 ception of the degree of accuracy possible and desirable is 

 requisite to save misapprehension and needless computa- 

 tion on the one hand, and to secure all attainable exact- 

 ness on the other hand. 



c Sandeman, ' Pelicotetics/ p. 214. 



d Leslie, ' Inquiry into the Nature of Heat,' p. 505. 



