OF PERIODIC PHENOMENA. 363 



stances, to fix the mean time of such periodic phe- 

 nomena with tolerable precision. In others, no 

 doubt, the date registered would be liable to consi- 

 derable alteration from observations continued over 

 a longer period of years. 



(26.) Each entry must be considered in itself 

 alone, and as an approximation more or less to the 

 mean period of occurrence of the phenomenon re- 

 corded, not as in necessary connection with any 

 other, with which, if so viewed, it may appear incon- 

 sistent, from the observations not being equally 

 extensive in the two cases. Thus, to illustrate our 

 meaning by a particular instance, there is of course 

 a mean time, with every species of bird, for laying 

 and hatching : if the true mean w r ere really deduced 

 and known, in respect of each of these two pheno- 

 mena, the interval between them would be the exact 

 period occupied in incubation. Looking, however, 

 at the present calendar, and without remembering 

 that each date is but an approximation to the true 

 one, this approximation being sometimes much 

 nearer than others, it may appear otherwise, and 

 this interval be thought much shorter than in reality 

 it is. 



(27.) This circumstance has suggested the pro- 

 priety of inserting the dates of the various pheno- 

 mena observed as they occurred in succession in 

 some one year, and without reference to obtaining a 

 mean time; by which arrangement, perhaps, their 

 true order of sequence can be more correctly traced. 

 And accordingly such a column in the calendar has 

 been annexed to the others already spoken of (24), 

 and set aside exclusively for the entries of a given 



R 2 



