Monthly Vaeiations for the Atmospheric Pressure. 



XClll 



The monthly mean diurnal range does not vary greatly in the six months for which the sun is south of 

 the equator, nor in the six months for which it is north of the equator ; thus, the mean range for the six 

 months October to March = 0-257 in., and for the six months April to September = 0-170 in. On the 

 whole, the diurnal range varies inversely with the monthly mean pressure. See No. 197 where the ranges of 

 the mean diurnal variations are considered. 



Table 86. — Variations of the Diurnal Range of Atmospheric Pressure, with reference to the 

 Moon's Age and Declination for the Years 1843-6. 















After 













Moon's 

 Age. 



1843. 



1844. 



1845. 



1846. 



Mean. 



Moon 

 farthest 

 North. 



1843. 



1844. 



1845. 



1846. 



Mean. 



d. d. 



in. 



in. 



in. 



in. 



in. 



d. d. 



in. 



in. 



in. 



in. 



in. 



14—16 



+ •012 



-•032 



-•063 



-•027 



-•027 



27— 1 



+ •031 



+ •019 



+ •026 



+ •022 



+ •025 



17—20 



-014 



•000 



+ •049 



•000 



+ •009 



2— 5 



+ •010 



+ •022 



-•006 



+ ■004 



+ •008 



21—24 



-•034 



+ •017 



+ •069 



+ ■030 



+ •021 



6— 8 



-■001 



+ •009 



-•005 



-■053 



-•012 



25—28 



+ •001 



-•001 



-•009 



-003 



-•003 



9 12 



+ ■010 



-■018 



-•021 



+ ■010 



-005 



29— 1 



+ •030 



+ •024 



+ •005 



+ •005 



+ •016 



13—15 



-•032 



-•026 



+ ■019 



+ •009 



-•008 



2— 5 



+ •010 



+ •011 



+ •002 



+ •019 



+ •011 



16—19 



-•028 



-•007 



-•003 



-•002 



-•010 



6— 9 



+ •001 



+ •026 



-•Oil +^008 



+ •008 



20—22 



+ •015 



-•015 



-•033 



-005 



-•010 



10—13 



-■007 



-•044 



-•045 -^033 



-•032 



23—26 



-•003 



+ •016 



+ •026 



+ ■019 



+ ■015 



194. Variation of the Diurnal Range of Atmospheric Pressure with the Moon's Age. — Investigations have 

 been entered into by different meteorologists for the purpose of exhibiting the effect of the varying position of the 

 moon upon the mean daily pressure of the atmosphere ; their success has been on the whole very doubtful. In 

 our latitudes it is not easy to extricate the laws of these variations on account of the magnitude of the irregular 

 changes ; it was for this reason that, after discussing the daily mean pressures for the year 1843 with refer- 

 ence to the lunar arguments, the discussion of the diurnal ranges was substituted for that of the daily means ; 

 as it was conceived that the variation of the diurnal range might be considerable (as in the case of the oceanic 

 tides, &c.), though the variation of the mean should be nearly or altogether zero ; such had been found to be 

 the case for the magnetic declination. The results of these discussions for each year, and for the mean of four 

 years, are given in the first part of Table 86. The results for the four years are wonderfully consistent, and 

 that of the mean of the four years may be expressed thus. — The diui'nal range of the barometer is a minimum 

 near opposition, and it is a maximum about the beginning of the second quarter, and immediately after con- 

 junction ; perhaps the intermediate minimum near conjunction is accidental and might disappear in a larger series. 

 The range of these mean numbers is very considerable, upwards of half-a-tenth of an inch, and it is probable 

 that had the means for single days of the argument been given, the range would have been nearly twice as 

 great. This result is wholly different from what we should have expected when comparing the oscillation of 

 the atmosphere with that of the ocean, and it appears difficult to offer an explanation for it ; we shall find how- 

 ever when we examine Table 91, that it is probably connected with the force of the wind; at least that obeys 

 the same law, the diurnal range of the barometer being greatest when the force of the wind is greatest. 



195. Variation of the Diurnal Range of the Atmospheric Pressure tuith Reference to the Moon's Decima- 

 tion. — The values for four years for this argument are given in the second part of Table 86 ; the results for 

 each year agree here also to a remarkable extent with that shewn by the mean of the whole four years. The 

 diurnal range of the barometer is a maximum when the moon is farthest north, it is a minimum when the 

 moon is south of the equator. This result is also connected vath. that for the force of the wind (see No. 201), 

 the diurnal range of the barometer being greatest when the force of the wind is greatest, and vice versa. 



