XVI 



General Results of the Makeestoun Observations. 



extent • the smaller disturbances not having been observed with the same completeness in that year. The 

 mean diurnal ranges for each month from these two Tables are as follow : — 



Jan. Feb. March. 



April. 



May. 



June. July. -Aug. Sept. 



Oct. Nov. 



Dec. 



Year. 



1844, ir-63 13'-63 19'-36 



19'-10 



14'-83 



12'-60 13'-36 16'-58 17'-74 



i9'-26 iQ'-ee 



12'-95 



15'-89 



1845, 17'-81 15'-31 16'-52 



17'01 



14'-67 



13'-82 13'-72 17'-79 18'-20 



14'-48 12'-64 



15'-01 



15'-58 



Mean, 14'-72 14'-47 l7'-94 18'-05 14'-75 13'-21 13'-54 17'-18 17'-97 16'-87 16'-15 13'-98 



15'-74 



These means give the same law of variation as that already found from Table 5, but the values are con- 

 siderably higher. From the means for both years we may conclude, that the mean angle, including the diurnal 

 oscillations in years of moderate disturbance at Makerstoun, is about 18' at the equinoxes, about 14' at the 

 solstices, and about 16' for the whole year. 



Annual Variation of the Ranges of the Monthly Mean Diurnal Variation. — We have considered above 

 the annual variation of the mean ranges for each day, we now give in Table 6 the ranges of the mean diurnal 

 variation for each month. The first four lines contain the diurnal ranges of the means of all the regular daily 

 observations made in each month ; and, as in Table 5, only 1844 and 1845 are comparable with each other: 

 the last line contains the range of the hourly means for each month, as deduced from the observations for the 

 4 years given in Table 12. 



13. From the last line of Table 6 it appears that when a sufficient number of observations is employed, the 

 range of the mean diurnal variation is nearly constant for the six months, April to September, being on tlie 

 whole rather larger for the first three than for the last three of that half-year : the mean range for the whole 

 six months is about ll'O. The range is nearly of equal value for pairs of the remaining six months, namely, 

 for March and October ( = 9'-2) ; for February and November ( = 7'-3) ; and for January and December ( = 5'-9). 





Table 6.- 



-Range 



's of the Mean Diurnal Variation 



of Magnetic Declination. 





Year. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



Mean. 



1843 

 1844 

 1845 

 1846 



Mean) 

 of all/ 



4-92 

 5-26 

 6-95 

 6-54 



5-85 



8-35 

 6-36 

 7-31 

 6-05 



7-11 



7-93 



9-94 



9-92 



10-56 



9-28 



10-52 

 10-20 

 13-08 

 12-58 



11-29 



9-93 



8-96 



12-42 



12-79 



11-02 



11-14 

 11-05 

 12-52 

 11-41 



11-61 



9-95 

 10-06 

 10-86 

 11-94 



10-60 



10-83 

 10-31 

 12-67 

 10-62 



10-83 



10-75 



9-95 



10-53 



11-99 



10-45 



7-90 



10-94 



9-42 



9-24 



9-50 



4-63 

 9-28 

 7-50 

 8-67 



7-55 



5-73 

 5-96 

 6-43 

 5-77 



6-02 



7-54 

 7-67 

 8-40 

 8-52 



8-03 



14. When we examine the range for each month of the mean diurnal variation as deduced from any single 

 year's observations, we observe a similar result to that obtained from the means of the diurnal ranges (Table 5) ; 

 namely, that the range is, on the whole, greater for the months near the equinoxes than for those near the 

 summer solstice ; but as we combine a larger number of observations the difference gradually disappears, till 

 (as we see in the mean of four years' observations, Table 6) the only difference appears in the slightly greater 

 range for the months immediately before the summer solstice than for those immediately after it. The differ- 

 ence betwixt the two results is, therefore, in all probability, due to irregular causes, which shift the epochs of 

 the extreme positions of the magnet. That this is the case will be rendered nearly evident by the ranges of the 

 diurnal variation as deduced from the 17 days' observations mth least irregular disturbance (see 1844, 

 p. 339, and p. 5 of this volume) ; they are as follow, 



.Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



3'-43 



4'-54 



7'-81 



ll'-55 



9'-66 



ll'-41 



ll'll 



ll'-18 



9'-64 



7-70 



5'-31 



4'- 11 



These, with the exception of the range for May, indicate generally the constancy of the regular diurnal 

 range in the months from April till August. The exception of May is easily explained by the fewness of the 

 observations, and the method by which the observations were selected (see volume for 1844, p. 339). 



15. Since, then, the means of the diurnal ranges differ from the diurnal ranges of the means, chiefly because of 

 irregular disturbing causes which shift the epochs of the extremes, the differences of the results will give some 

 measure of these disturbances. Taking the differences betwixt the last line of Table 5, and the last of Table 6, 

 we have, 



.Tan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



3'-58 



3'-53 



4'-27 



3'-87 



2'-29 



l'-36 



2'- 51 



3' 50 



4'-94 



3'- 6 6 



3'-21 



3'-46 



