336 



Results of the Makeestoun Observations, 1844. 



Variations of the Diurnal Bangcs of Magnetic Declination, with reference to the Moon's Age. — The 

 differences of the (liurnal ranges of the magnetic declination at the different ages of tlie moon are very remark- 

 •Me ; these, and tlie law of the variations, are shewn more regularly in the following means of groups : — 



14 davs to 16 davs, Full Moon, 16'-22 



17 .'..... 20 ".. 15'-90 



21 24 ... li'-59 



25 28 ... 10'-13 



From these means, the diurnal range of magnetic declination is greatest about full moon, and least about new 

 moon, the actual epochs heing rather before these events. This law is exceedingly well marked ; in fact the 

 variation of these means is upwards of seven minutes, and is greater than the variation of the monthly means. 

 This result is not a little curious, when it is remembered that each of the 8 values above is a mean of the 

 ranges for from 36 to 48 days ; these, being distributed in groups of 3 or 4 days at equal intervals over the 

 year, the range due to the sun's position alone is completely eliminated. In 1844, then, the diurnal range of 

 magnetic declination varied more in the means for 8 periods during the synodical revolution of the moon, than 

 in the means for 12 periods during the earth's revolution round the sun. 



It should be remarked, that the diurnal range does not vary much during the first and fourth quarters, 

 nor during the second and third quarters, the average diurnal range of the 10 days from the moon's age, 10 

 to 19 davs, being 16'-89 ; and for the 10 days from the moon's age, 25 days to 4 days, being 10'-83 ; the 

 mean for the remaining 10 days (20 to 24, and 5 to 9) heing 12'-70. 



An examination of the simple diurnal ranges, shews that the law annoimced is distinctly marked in four 

 or five lunations, as may be readily seen in the projected values.* It is well marked in the lunation 

 occurring between January 19 and February 17; but the variations of the range are greater in the two 

 lunations immediately succeeding that and in the two lunations occurring in September and October. If the 

 points which indicate the weekly means in the projections be followed, it will be evident that the law is still 

 existing, though with more ii-regularity, in the months of August, November, and December. The value of the 

 range oscillates considerably ft-om day to day, but in the lunations particularly refeiTcd to, the larger values, 

 as well as the smaller values, and weekly means, equally exhibit the same law ; in some cases, intermittent 

 disturbances produce irregidarity. It is evident, from these projected values, that the diurnal range of the 

 magnetic declination in 1844 was greatest ivhen the sun and moon were in opposition near the equator, or, 

 more accurately, immediately before the vernal and after the autumnal equinox. 



Variations of the Diurnal Range of Magnetic Declination, with reference to the Moons Declination. — 

 From the second portion of Table IV., it appears that the diurnal range is a minimum when the Moon has 

 its gi-eatest south and greatest north declinations, and a maximum about 2 days before it passes the equator 

 moving south, aud 2 days before it passes the equator moving north. The following are means of groups : — 



13 days to 15 days, Moon farthest south, 12'-05 



16 19 ... 12'-53 



20 22 ... 13'-16 



23 26 ... 16'-08 



The variation of these means is still considerable, being nearly 5'. The law of the variation of the diurnal 

 ranges for 1844, is nearlv the same with reference to the moon's position in declination as to the sun's position 

 in declination, and may be generally stated thus : — The diurnal range of magnetic declination is less when the 

 sun or moon has its greatest north or south declination, than at the intermediate periods. 



It will be remarked, that this result may be cliiefly or altogether dependent on the other law given above, 

 with respect to the relative positions of the sun and moon. 



Variations of the Diurnal Ranges of Magnetic Declination with reference to the Moon's Distance from the 

 Earth.— Apogee happens nearly at the time of greatest north declination in 1844; consequently, from 

 the previous discussion, there are minima about apogee and perigee, and maxima at the intermediate periods ; 

 as is also evident in the following means of groups : — 



6 days after apogee to 6 days before perigee, 15''26 

 5 days to 2 days before perigee, . 13'-52 



1 day before to 1 day atler perigee, . 13'-29 



2 days to 5 days after perigee, . . 12'- 80 



6 days after perigee to 6 days before apogee, 13'-34 

 5 days to 2 days before apogee, . . 16''25 



1 day before to 1 day after apogee, 11''90 



2 days to 5 days after apogee, . 12'-03 



* See the Plates at the end of the volume. 



