316 THE PRINCIPLES OF SCIENCE. 



Lexell's Comet it was afterwards shown, curiously enough, 

 that its appearance was not one of a regular series of 

 periodical returns within the sphere of our vision, but a 

 single exceptional visit never to be repeated, and probably 

 due to the perturbing powers of Jupiter. Yet this soli- 

 tary visit was a strong confirmation of the law of gravity 

 with which it seemed to be in conflict. 



The division of Biela's Comet into two companion comets 

 was at the time when it occurred one of those urilooked- 

 for and inexplicable events which awaken the attention 

 and interest of observers in the highest degree. Comets 

 indeed have altogether the character of eccentric strangers 

 intruding into our planetary system, and in almost every 

 point they are yet inexplicable ; but there is a possibility 

 that the separation of Biela's Comet may prove to be a 

 comparatively ordinary event of cometary history. For 

 if, as is now believed, comets be aggregates of small me- 

 teoric stones or particles, forming the denser parts of con- 

 tinuous streams of such bodies circulating round the sun, 

 then it is not unlikely that these aggregates may at times 

 be increased or diminished by the meeting or separation 

 of meteoric streams. 



Singular Exceptions. 



Among the most interesting of apparent exceptions are 

 those which I propose to call singular exceptions, because 

 they are more or less closely analogous to the singular 

 cases, or solutions which occur in mathematical science. 

 A general mathematical law embraces an infinite multi- 

 tude of cases which have a perfect agreement with each 

 other in a certain respect. It may nevertheless happen 

 that a single case, while obeying the general law, stands 

 out as apparently different from all the rest. The daily 

 rotation of the earth upon its axis gives to all the stars 



