METEOROLOGICAL OBSERVATIONS AT PLYMOUTH. 



165 



annual pressure of any hour does not differ more than about 

 •01 from the mean annual pressure of the twenty-four hours. 



We may lience render any past register at Plymouth avail- 

 able in deducing the mean pressure, by applying, according to 

 the signs, the corrections given in the preceding table. 



Table XX. 



Showing the Pairs of Hours, the Mean Pressure of which ap- 

 proaches nearest the Mean of the Twenty-four Hours. 



Pairs of 

 Hours. 



8 A.M. 



and 

 6 P.M. 



3 A.M. 



and 

 8 P.M. 



9 A.M. 



and 

 1 P.M. 



3 P.M. 

 and 



10 A.M. 



4 A.M. 



and 

 10 A.M. 



9 A.M. 



and 

 5 P.M. 



2 P.M. 



and 

 8 P.M. 



11 A.M. 



and 

 6 P.M. 



2 P.M. 



and 

 9 P.M. 



6 A.M. 

 and 



7 P.M. 



Deviations 



•0001 + 



•0003+ 



•0003+ 



•0004+ 



•0005- 



•0006- 



•0007- 



•0008+ 



•0009+ 



•0010- 



The mean pressure therefore at Plymouth may be obtained 

 by observations at either of the pairs of hours above given. 



It may be further seen by Table I. that the mean maximum 

 and mean minimum pressures are 29*8080, and 29*7911, giving 

 a mean oscillation = *0169, or very nearly the same as that 

 = '01688 already deduced from the four same oscillations, 

 Table VI. 



It is not therefore unimportant to determine the hours which 

 either give exactly or come nearest the mean maxima and mi- 

 nima of the twenty-four. 



Now the pressure at 3 p.m. differs extremely little from the 

 mean minimum, the difference being only -0003, whilst the 

 mean pressure at 11 p.m. approaches nearest the mean maxi- 

 mum, the difference being only '0012. If we had taken these 

 observations at these hours as the mean maxima and minima of 

 the twenty-four, we should have had for the mean oscillation 

 •0184, which would have differed from the true mean oscilla- 

 tion by -0015, and for the mean pressure 29*8000, which would 

 have been very near the true mean pressure. 



The mean results given in Table I. enable us to find the 

 single hours, or pairs of hours, which give either singly or 

 combined the mean maxima and minima pressures, or approach 

 nearest to them. These are given in the next table. 



