Annual Vaeiations foe the Horizontal Component of Magnetic Foece, xxxv 



60. Annual Variation of the Diurnal Range of the Horizontal Component of Magnetic Force. — The means 

 for 1844 and 1845 only, in Table 26, are comparable with each other. From the last line of Table 26 we find 

 that the mean daily range was least in the months of December, January, and February, and less in May and 

 June than in April, July, and August. These mean ranges are deduced from the ordinary daily observations. 

 \Mien we seek for the absolute ranges, as obtained from all the extra observations made in the years 1844 and 

 1845 (Table LX. 1844, p. 400, and Table L. p. 28 of this volume), we obtain the following numbers (prefix 

 0-00) :— 





Jan. 



Feb. 



March. 



Ajiril. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



1844, 



2943 



3811 



6193 



6234 



5088 



3802 



4538 



5319 



4538 



5134 



6556 



2965 



1845, 



5758 



3009 



4268 



6138 



4733 



4054 



4174 



5270 



5062 



3759 



2864 



3995 



Mean, 4355 3410 5230 6186 4910 3928 4356 5294 4800 4446 4710 3480 



These numbers follow the same law as those in Table 26, they are, however, considerably larger ; the in- 

 crease is most marked in the Avinter months. From the means of both years we may conclude that the mean 

 value of the diurnal change of the horizontal component of magnetic force at Makerstoun in years of moderate 

 disturbance is about 0*0057 in April and August, and about 00038 at the solstices, the whole horizontal com- 

 ponent being unity. 



Table 27- — Ranges of the Mean Diurnal Variation of the Horizontal Component of Magnetic Force. 



Year. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



Mean. 



1843 

 1844 

 1845 

 1846 



Mean) 

 of aUj 



0-00 



0928 

 0690 

 1742 

 1027 



1062 



O'OO 



0862 

 0875 

 1008 

 0956 



0840 



0-00 



1674 

 2195 

 2247 

 2333 



2185 



0-00 



3209 

 3378 

 3585 

 4099 



3482 



0-00 



3409 



3644 



3623 



5504 



3969 



0-00 



3615 

 3179 

 3458 

 4822 



3685 



0-00 



3867 

 3657 

 3651 

 5550 



4148 



000 



3541 

 3501 

 3374 

 5573 



3997 



0-00 



2698 



2948 



3282 



4427 



3100 



0-00 



2203 

 2313 

 1877 

 2395 



2169 



o-ou 

 1051 

 1305 

 1537 

 2392 



1431 



0.00 



0724 

 0760 

 1845 

 1099 



0897 



0-00 



2128 

 2212 

 2387 

 3152 



2452 



61. Annual Variation of the Ranges of the Monthly Mean Diurnal Variation. — From the last line of Table 

 27, it appears that the range of the monthly mean diurnal variation is least in December, January, and February, 

 and that it is less in June than in May, July, or August, The following are the diurnal ranges of the monthly 

 mean diurnal variations, as deduced from the 20 days selected as free from irregular disturbance in the 

 years 1844 and 1845 :— 



Prefix. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



0-00 



0547 



0682 



1833 



3154 



3279 



3209 



3417 



3080 



2988 



2030 



1291 



0875 



62. When we examine the mean diurnal ranges as deduced from all the observations (extra and ordinary) made 

 in 1844 and 1845, we find that the means for March and April, and for August and September, are greater 

 than the means for May, June, and July ; when we deduce the mean diurnal range from the regular daily 

 observations, as in Table 26, then we find only the mean for April greater than the means for May, June, and July, 

 the means for July and August being greater than those for May and Jime. So when we consider the range of 

 the monthly mean variation as in Table 27, we find the mean for June less than the means for May, July and 

 August only ; and, finally, in the ranges last given, deduced from the mean variations of undisturbed days, the 

 differences for the months from April to September almost altogether disappear, the excess for July being in 

 all probability accidental. This result is similar to that already found for the magnetic declination, and we may 

 draw from it a similar conclusion, that the excess of the diurnal range in the equinoctial months over that 

 for the midsummer months is due to irregular disturbance. 



63. Annual Variation of the Mean Difference of a Single Observation of the Horizontal Component from the 

 Monthly Mean at the corresponding Hour. — The mean differences for 1844 and 1845 (from Table XXXIV., 

 1844, p. 368, and Table XXX., p. 17, of this volume) are as follow (Prefix 0-000) :— 



Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 



1844, 350 533 823 770 568 400 469 645 587 689 591 521 



1845, 697 515 477 650 503 421 447 577 610 473 501 699 



Mean, 523 524 650 710 535 410 458 611 598 581 546 610 



Mean. 



579 



547 



563 



MAG. AND MET. OBS. 1845 AND 1846. 



