ISO ESSAY ON PROBABILITIES. 



by which is meant, not that general facility of misper- 

 ception which is common to all the human race, but 

 that particular habit or temperament which causes some 

 to differ from persons in general in their method of 

 perceiving. Thus it will frequently happen that when 

 two observers note the time of a phenomenon, by the 

 same watch,, one will always see the event, or imagine 

 he sees it, before the other does the same. This 

 personal error, wbich is seldom large, is beginning to 

 receive the attention of observers. It is not perceptible 

 as long as the natural data of a science remain imper- 

 fectly known, being mixed up and lost in errors of 

 greater magnitude ; but it produces discoverable effects 

 so soon as the science approaches towards accuracy. 



3. From fixed sources of error peculiar either to 

 the species of apparatus employed, or to the individual 

 instrument with which the observations are made. 

 This answers precisely to the personal error of the 

 observer in its effects : it matters nothing whether the 

 clock be one second too fast, or the observer, in the 

 result of the observation. 



4. From the imperfection of human senses and in- 

 struments. To note a measurable phenomenon without 

 any error at all, would require sight and touch by 

 which every magnitude, however small, could be per- 

 ceived and correctly estimated. Such senses belong to 

 no one, and the degree of approach towards perfection 

 not only varies with the observer, but is different at 

 different times with the same observer. Many errors 

 to which instruments are subject ought in strictness to 

 be classed under the first head ; if, for instance,, an 

 astronomical circle gradually change its form, or 

 undergo daily expansion and contraction by variations 

 of temperature, the diversity of results which such a 

 piece of brass will shew are certainly subject to laws, 

 and might be predicted, if we possessed sufficient 

 knowledge of the constitution of the metal, and the 

 laws which regulate the effect of pressure, temperature, 

 moisture, &c. upon it. But so long as such laws are 



