1877.] the Lines of equal Barometric Pressure. 519 



highest in 1847 and 1849, the least pressure being in 1848. There is, 

 however, an appearance of systematic change in the yearly mean values 

 of /3^, /S^, and of Q, which may be noticed on account of its partial coinci- 

 dence with the decennial period of sun-spots and magnetic variations. 



The differences of pressure shown by the isobars passing through the 

 three stations was least in 1844, and greatest in 1847-48*. It does not 

 follow, however, that the tenth-inch isobars were really further separate 

 in 1844 than in 1847-48, since this depends also on the perpendicular 

 distance between the isobars, or on the angle 6 which they make with 

 the meridian ; thus we find the perpendicular distances of the tenth- 

 inch isobars in geographical miles in the different years to be as follows : — 



1842. 1843. 1844. 1845. 1846. 1847. 1848. 1849. 

 218 273 317 242 273 266 281 330 



Thus the isobars were furthest separate in 1844 and 1849, and nearest 

 in 1842 [and 1845. The regular variation of and /S^ was thus chiefly 

 due to the inclination of the isobars to the meridian. From the last 

 line of Table lY. we see that the isobars were from 10° north of west 

 (W. by N. nearly) in. 1844, and from 19° south of west (W.S.W. nearly) 

 in 1848. Though there is a coincidence in these epochs wdth those of 

 minimum and maximum sun-spot frequency, it is a coincidence which 

 requires confirmation by a longer series of observations, the more espe- 

 cially as the chauge of value of 6 from 1848 to 1849 is very considerable. 



Annual Variations. — The following Table contains the monthly mean 

 values for the whole eight years, those of 6 being derived from the mean 

 values of and /S^ by equation (4) (they are therefore not the means of 

 the quantities in Table TV.). and /d^, with their probable errors, are 

 in thousandths of an iuch. 



Table Y. — Monthly Means from eight years' Observations. 



Month. 



Mean. 

 Green- 

 wich, 

 -h29. 



A- 



Pro- 

 bable 

 error 

 of/3,. 





Pro- 

 bable 

 error 

 of Q^. 



0. 



Pro- 

 bable 

 error 

 of 0. 



/3 = 0-l in. 





in. 













o 



miles. 





0-949 



4-163 



+20 



92 



+ 14 



261 



+ 9 



157 





0-921 



-hl33 



26 



54 



19 



271 



13 



186 





0-923 



+ 96 



15 



42 



15 



271 



12 



254 





0-880 



4- 28 



22 



11 



15 



275 



25 



839 





0-964 



-f 12 



21 



14 



20 



216 



25 



1710 





0-966 



-f 85 



8 



36 



8 



272 



5 



285 



July 



0-978 



+ 110 



9 



46 



7 



273 



3 



220 





0-958 



+ 93 



9 



29 



9 



280 



5 



220 



September 



1-004 



+ 33 



17 



1 



9 



295 



15 



546 





0-870 



+ 110 



11 



55 



12 



267 



7 



238 





0-905 



+ 138 



12 



100 



9 



249 



7 



186 





1-026 



+ 133 



31 



128 



23 



230 



13 



178 





0-945 



+ 95 



4 



+50 



4 



264 



2 



267 



* This result was obtained for (i-^ from the Grreenwich and Makerstoun Observa- 

 tions in 1850 (Trans. Eoy. Soc. Edinb. xix. part 2, p. xci). 



