METEOROLOGICAL OBSERVATIONS AT PLYMOUTH. 159 



20, Thus the results given in Table XII. cannot be regarded 

 as strictly comparable, the observations having been in some in- 

 stances confined to short periods of time, also to different sea- 

 sons, and to certain portions of the day, in which the difference 

 from the mean oscillation of the twenty-four hours has never 

 yet been ascertained. In order to make such observations 

 completely available, it is requisite to determine these differ- 

 ences in different places, and at different altitudes above the 

 sea, not only for a mean year, but for different seasons, and 

 even months. We have not, however, at present, data com- 

 petent to such deductions. 



21. I have arranged in the three following tables such inform- 

 ation on this point as is derivable from the extensive series of 

 hourly observations now under consideration, and which will 

 be found to contain the amount of the four semi-oscillations in 

 different years and seasons, with their various differences from 

 the mean of each. 



Table XIII. 



Showing the four Semi-Oscillations of the Years 1837, 1838, 

 1839, with the Difference of each from the Mean Oscillation 

 of the Year, together with the Differences of the Two 

 Ascending and the Two Descending Oscillations. 



Years. 



i 



Morning Tides. 



Evening Tides. 



Ascending + 



Descending — 



Ascending + 



Descending — 



1 to 10 A.M. 



Differ- 

 ence. 



10 A.M. 

 to 4 P.M. 



Differ, 

 ence. 



4 to 10 P.M. 



Differ- 

 ence. 



10 P.M. 



to 4 A.M. 



Differ- 

 ence. 



1837 

 1838 

 1839 



•01800 

 •01G40 

 •01625j 



•0126 

 •0138 

 •0137 



•0054- 

 •0026- 

 •0025- 



•0174 

 •0170 

 •0155 



•0006 - 

 •0006 + 

 •0007 - 



•0234 

 •0190 

 •0188 



•0054+ 

 •0026+ 

 •0025+ 



•0186 

 •0158 

 •0170 



•0006 + 

 •0006 - 

 •0007 + 



Mean 



•01688^ -01336 



•00352- 



•01663 



•00025- 



•0204 



•00352 ^01713 



•00025+ 







Years. 



Ascending + 



Descending — 





Differences of a.m. and p.m. 



r 18371 

 \ 1838 [ 

 1 1839 J 



•0108 

 •0052 

 •0051 



•0012 

 •0012 

 •0015 



Mean 



•007 



•0005 



1 

























