= 
_— 
ANNUAL VARIATIONS FOR THE HORIZONTAL COMPONENT OF MAGNETIC FoRCE. 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. 
When 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. April. May. June. July. Aug. Sept. Oct. Noy. Dec. 
1844, 2943 3811 6193 6234 5088 3802 4538 5319 4588 5134 6556 2965 
1845, 5758 3009 4268 61388 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 winter 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 0-0088 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. 
ee 
I 
Year. Jan. Feb. | March.| April. | May. | June. | July. | Aug. | Sept. Oct. Nov. | Dec. || Mean. 
0-00 0-00 0-00 0-00 0-00 0:00 0-00 0-00 “0-00 0-00 0-00 0.00 0-00 
1843 || 0928 | 0862 | 1674 | 3209 | 3409 | 3615 | 3867 | 3541 | 2698 | 2203 | 1051 | 0724 || 2128 
1844 | 0690 | 0875 | 2195 | 3378 | 3644 | 3179 | 3657 | 3501 | 2948 | 2313 | 1305 | 0760 || 2212 
1845 || 1742 | 1008 | 2247 | 3585 | 3623 | 3458 | 3651 | 3374 | 3282 | 1877 | 1537 | 1845 || 2387 
1846 | 1027 | 0956 | 2333 | 4099 | 5504 | 4822 | 5550 | 5573 | 4427 | 2395 | 2392 | 1099 || 3152 
ay 1062 | 0840 | 2185 | 3482 | 3969 | 3685 | 4148 | 3997 | 3100 | 2169 | 1431 | 0897 | 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 93417 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 June. 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. |; Mean. 
1844, 350 533 823 770 568 400 469 645 587 689 591 521 579 
1845, 697 515 477 650 503 421 447 577 610 473 501 699 547 
Mean, 523 524 650 710 535 410 458 £611 598 581 546 610 563 
MAG. AND MET. oBS. 1845 ann 1846. a 
