354 



Results of Makerstoun Observations, 1844. 



TABLE XXI. — Number of differences in 1000 (without reference to sign), which occur between 

 the limits of successive minutes in each hour of 1844. 



Mak. 

 M. T. 



0' 

 to 

 1'. 



1' 

 to 



2'. 



2' 

 to 

 3'. 



3' 



to 

 4'. 



4' 

 to 

 5'. 



5' 

 to 

 6'. 



6' 

 to 



7'. 



7' 

 to 

 8'. 



8' 

 to 

 9'. 



9' 

 to 

 10'. 



10' 

 to 

 15'. 



15' 

 to 

 20'. 



20' 

 to 

 25'. 



25' 

 to 

 30/ 



30' 

 to 

 40'. 



h. 



12 

 13 



14 



15 



16 



17 



18 



19 



20 



21 



22 



23 







1 



2 



3 



4 



5 



6 



7 



8 



9 



10 



11 



427 

 455 

 510 

 446 

 525 

 513 

 519 

 484 

 497 

 433 

 465 

 436 

 481 

 494 

 411 

 408 

 510 

 541 

 455 

 427 

 338 

 363 

 369 

 344 



264 

 280 

 220 

 261 

 210 

 248 

 248 

 306 

 242 

 299 

 299 

 312 

 264 

 261 

 341 

 373 

 280 

 239 

 213 

 264 

 242 

 309 

 341 

 264 



156 

 99 

 124 

 118 

 137 

 111 

 124 

 127 

 150 

 150 

 137 

 153 

 150 

 127 

 137 

 89 

 105 

 99 

 115 

 150 

 175 

 153 

 115 

 191 



48 

 45 

 38 

 76 

 51 

 48 

 45 

 22 

 51 

 61 

 54 

 57 

 54 

 80 

 61 

 38 

 32 

 54 

 92 

 57 

 102 

 61 

 64 

 92 



25 

 35 

 35 

 38 

 13 

 25 

 6 

 29 

 16 

 22 

 13 

 16 

 16 

 19 

 16 

 51 

 19 

 25 

 38 

 35 

 54 

 22 

 29 

 45 



13 



25 

 19 

 16 

 16 

 16 

 29 

 13 

 16 

 3 



6 

 10 



13 

 16 

 25 

 6 

 19 

 19 

 13 

 35 

 25 

 22 



19 

 10 

 16 

 16 

 16 

 10 

 6 



16 



3 



19 



10 



3 



10 



6 



3 



6 



13 



16 



13 



16 



19 



16 



13 



13 



6 



13 



3 

 3 

 3 

 3 



6 

 3 

 3 

 3 



16 

 10 

 10 



16 

 6 

 16 

 10 

 10 



10 



19 



3 



6 



3 



6 

 6 



3 



6 

 3 



3 

 3 

 3 

 3 



6 



10 



3 



6 



3 



13 



3 



i"o 



3 

 6 



3 

 3 



3 

 3 

 3 

 3 

 3 

 6 

 10 

 3 



10 

 6 

 6 

 3 



16 



19 

 6 

 3 

 6 



13 

 6 

 3 



3 



3 



6 



19 



19 



19 



10 



10 



6 



6 

 3 

 3 



3 

 6 



6 



3 



3 

 6 

 3 

 3 

 6 



3 

 3 



3 



3 



3 

 3 



3 



3 

 3 



6 

 6 



6 



Note on the Diurnal Variation of the Magnetic Declination. — It is obvious from the pi'evious investiga- 

 tions that the diurnal variation is a compound phenomenon ; in its observed form it presents a curve with two 

 maxima and two minima, the principal maximum occurring immediately after noon, the secondary maximum 

 shortly after midnight, and the principal minimum about 10 h p.m. or 6 h a.m., at the former if the amount of 

 disturbance for the year be considerable. When the diurnal variation is considered in days nearly free from 

 intermittent disturbance, it is found that the minimum about 10 b p.m. almost wholly disappears, and the mini- 

 mum about 6 h a.m. is increased. Although, however, the 10 h p.m. minimum and the secondary maximum 

 wholly disappear, the diurnal curve, even when unaffected by disturbance, does not become a regular curve of 

 two branches ; on the contrary, there is still evidence of the action of a secondary or superposed cause of varia- 

 tion in the flattening of the curve from about 6 h p.m. till about 2 h a.m., in the sudden check in the rate of the 

 easterly progression about 6 h p.m., and its sudden increase again about 2 h a.m. These epochs, it will be 

 observed, are the times of the principal minimum and maximum for our perioeci. We might suppose the diur- 

 nal variation, therefore, when unaffected by intermittent disturbances, due to two operations of the same cause, 

 a direct action proper to one side of a zone producing a regular motion having its greatest westerly limit after 

 noon, and its most easterly limit about 6 h a.m. ; and an indirect or reflected action upon the opposite side of 

 the same zone producing similar motions simultaneously, but of less magnitude. Such a hypothesis, which has 

 an analogy to that of tides (say in the atmosphere), would satisfy the simplest form of the diurnal curve for 

 the year, and for all the months for which the sun is north of the equator ; another element perhaps requires 

 consideration for the winter months. 



The form of the diurnal curve is different at different seasons of the year. Colonel Sabine has shewn, in 

 his discussion of the St Helena Observations, that the diurnal curve has two opposite forms at St Helena, 

 according as the sun is to the north or south of the equator ; the one corresponding to the diurnal motion for 

 the northern hemisphere, the other to that for the southern hemisphere. An examination of the Makerstoun 

 Observations will shew an equivalent fact. During the months that the sun is north of the equator the form 

 and range of the diurnal curve have but little variation, but they change considerably in the months for which 

 the sun is south of the equator. If we take the means of the 10 days of least disturbance in the two months 



