Magnetical Observatory of Dublin during the Years 1840-43. 

 A = 332° 27'.27 + 6'.06 x n + AD; 



91 



n denoting, as before, the number of years reckoned from January 1 1840 



20. We have already seen that the diurnal changes of declination and tem- 

 perature are related in a very remarkable manner ; and we should, therefore 

 naturally be led to expect a corresponding relation in the annual changes of 

 the same elements. For the purpose of exhibiting it, I subjoin the diiferences 

 between the mean temperatures of each month, and that of the entire year as 

 deduced from the observations made at the Magnetical Observatoiy during 'the 

 years 1841-46. ^ ^ 



The numbers of this Table ^re projected in Plate HI. fig. 7, immediately 

 below the corresponding curve of declination, the scale being -05 of an inch to 

 one degree of temperature. It will be seen, on a comparison of the two curves 

 that the annual variations of the declination and temperature present the most' 

 complete accordance, not only as to the hours o^ raaa^imu and minima, but also 

 m their entire course.* There is a slight, but systematic difference in the 

 epochs of the ^.a^ values, those of the declination taking place about a 

 fortnight later than those of the temperature. This is what we should be led 

 d prion to expect, on the assumption that the magnetic changes are due to the 



ch.n ^'°T.*'1' '"'" "'' """'"' ' ^'^' '^"'^'^ '''^' *^^ correspondence between the annual 



result of the Stockholm observations. The correspondence thus traced, however, does not extend 

 beyond the fact, that the epochs of greatest and least declination coincide, nearly, with those of 

 greatest andl^st temperature ; and, in fact, the results themselves, although the means of obser- 

 vations extendmg over the space of thirty years (1786-1815), are manifestly encumbered with 

 too large an amount of observation error, to render any more detailed comparison possible. The 

 same remark applies, yet more strongly, to the results of the Manheim observations, as quoted by 

 K^MTZ. In both of these cases, the most ea.i.r/y position of the needle occurs at the time of 

 grecUeet temperature of the year, and vice versa. 



n2 



