122 DISCOURSE ON THE STUDY 



liquid about to be thrown away. Chemistry is full 

 of such incidents. 



(114.) In transient phenomena, if the number of 

 particulars be great, and the time to observe them 

 short, we must consult our memory before they 

 have had time to fade, or refresh it by placing our- 

 selves as nearly as possible in the same circum- 

 stances again ; go back to the spot, for instance, and 

 try the words of our statement by appeal to all re- 

 maining indications, &c. This is most especially 

 necessary where we have not observed ourselves, 

 but only collect and record the observations of others, 

 particularly of illiterate or prejudiced persons, on any 

 rare phenomenon, such as the passing of a great 

 meteor, the fall of a stone from the sky, the 

 shock of an earthquake, an extraordinary hail- 

 storm, &c. 



(115.) In all cases which admit of numeration or 

 measurement, it is of the utmost consequence to 

 obtain precise numerical statements, whether in the 

 measure of time, space, or quantity of any kind. To 

 omit this, is, in the first place, to expose ourselves 

 to illusions of sense which may lead to the grossest 

 errors. Thus, in alpine countries, we are constantly 

 deceived in heights and distances; and when we have 

 overcome the first impression which leads us to 

 under-estimate them, we are then hardly less apt to 

 run into the opposite extreme. But it is not merely 

 in preserving us from exaggerated impressions that 

 numerical precision is desirable. It is the very soul 

 of science ; and its attainment affords the only cri- 

 terion, or at least the best, of the truth of theories, 

 and the correctness of experiments. Thus, it was 



