MKTEOROLOGIGAL OBSERVATIONS AT PLYMOUTH. 153 



26,^280 hourly observations, in the three years above men- 

 tioned, appears to be 29,800 very nearly. 



7. The barometric curve, indicating the atmospheric tides, is 

 on this line four times in the day, viz. at I'SO a.m., at 7 a.m., at 

 mid-day, or 12 noon, and at 6'30 p.m. 



The pressure, having reached a minimum at 4 a.m., continues 

 to ascend for two hours, when it crosses the hne of mean press- 

 ure at 7 A.M., and ascending for the next three hours, reaches 

 its maximum at 10. 



This morning tide* begins now to recede, and again de- 

 scending for two hours crosses the line of mean pressure at 12 

 or mid-day ; from which, continuing to descend for four hours 

 more, it reaches a minimum at 4. The evening tide begins now 

 to show itself; it ascends for two hours and a half, being on 

 the line of mean pressure at 6'30 p.m., whence, continuing its 

 ascent for three hours and a half, it attains its maximum at 

 10 P.M. At this point it again turns, and descending for three 

 hours and a half, reaches the point of mean pressure between 

 12 and 1 a.m., whence it passes in the succeeding two hours 

 and a half to its minimum at 4. 



There are consequently, as in the afflux and reflux of the 

 sea, two tides in twenty-four hours. 



8. The principal elements of these tides are as follows : 



Table V. 



Morning. 

 Min. 297928 at 4 A.M. 



Max. 29-8061 at 10 A.M. 



Mean 297999 at 7 A.M. 

 ^tfo^'fS'?^' IoIm: } -0^33 Tin,e 6 hours. 

 ^ P^rlssuVe^l.!..™. } '""^l Time 3 hours. 

 ^PressuJ;'".l. °.*!..?!!!.!.'! } '^^^^ 'T''"^ 3 hours. 

 Noon. 

 Time of Mean Pressure, 12. 



Min. 297895 at 4 P.M. 

 Descending Semi-Oscil. "t 

 lation from 10 A.M. }■ -0166 Time 6 hours, 

 to 4 KM J 



Above Line of Mean 1 

 Pressure J 



•0062 Time 2 hours. 



^ p^Lu^"!...?.'!...!!!!.^" } ■">"'' '^''"^ ^ ^°"''- 



Evening. 



Min. 297895 at 4 P.M. 



Max. 29-8099 at 10 P.M. 



Mean 29-7999 at 6-30 P.M. 

 Ascending Semi.Oscilla- 1 ..„. . rT,.^„ ^ , „ 



tion from 4 to 10 P.M. > "204 Time 6 hours. 

 Below Line of Mean\ .„,„,™. „, , 



Pressure J 0104Time2i hours. 



Above Line of Meani .„,nn'r-™ oiu 

 Pressure / 0100 Time S^hours. 



Night. 

 Time of Mean Pressure, 1-30 A.M. 

 Min. 29-7928 at 4 A.M. 

 Descending Semi-Oscil- 

 lation, from 10 P.M. 5- -0171 Time 6 hours. 



to4 A.M 



Above Line of Mean " 



icil. I 

 .M. V 



'pTefsurl"!...!..!"!!!!'} •««71 Time2ihours. 

 ^P?rssu^e"!...?L.!!!.!!!} -OlOOTimeSihours. 



9. We may distinguish in these results, as delineated in 

 Plate v., four semi-waves, one between 4 and 10 a.m., a second 

 between 10 a.m. and 4 p.m., one between 4 p.m. and 10 p.m., 

 succeeded by one between 10 p.m. and 4 a.m., being precisely 



* By the term tide I mean the course of the barometric wave, from the 

 period of its ascent to the termination of its descent, that is, from minimum to 

 minimum. 



