MAGNETIC VARIATION. 119 



ton in Scotland, Washington, and Toronto, soon after 10 or 

 11 P.M. 



The four movements of the needle, which I recognized in 

 1805,* have been represented in the admirable collection of 

 observations made at Greenwich in the years 1845, 1846, 

 and 1847, as the results of many thousand horary observa- 

 tions in the following four turning points, j namely, the first 



* Sea extracts from a letter, which I addressed to Karsten, from 

 Rome, June 22, 1805, "On four motions of the magnetic needle, 

 constituting, as it were, four periods of magnetic ebbing and flowing, 

 analogous to the barometrical periods." This communication was 

 printed in Hansteen's Magnetismus der Erde, 1819, s. 459. On the 

 long-disregarded nocturnal alterations of variation, see Faraday, On 

 the Night Episode, 3012-3024. 



f Airy, Magnetic and Meteorological Observations made at Greenwich 

 (Results, 1845, p. 6; 1846, p. 94; 1847, p. 236). The close correspond- 

 ence between the earliest results of the nocturnal and diurnal turning 

 hours, and those which were obtained four years later, in the admi- 

 rable observatories at Greenwich and at Toronto, in Canada, is clearly 

 shown by the investigation made by my old friend Enke, the distin- 

 guished director of the observatory at Berlin, between the correspond- 

 ing observations of Berlin and Breslau. He wrote as follows on the 

 llth of October, 1836: "In reference to the nocturnal maximum, or 

 the inflection of the curve of horary variation, I do not think that 

 there can be a doubt, as, indeed, Dove has also shown from the Frei- 

 berg observations for 1830 (Poggend., Ann., bd. xix., s. 373). Graph- 

 ical representations are preferable to numerical tables for affording a 

 correct insight into this phenomenon. In the former great irregular- 

 ities at once attract the attention, and enable the observer to draw a 

 line of average ; while in the latter the eye is frequently deceived, and 

 individual and striking irregularities arc mistaken for a true maximum 

 or minimum. The periods seem to fall regularly at the folloAving 

 turning hours : 



The greatest eastern declination falls at S A.M., 1 maximum E. 



The greatest western declination falls at 1 P.M., 1 minimum 'E. 



Tie secondary or lesser eastern maximum falls at . 10 P.M., 11 maximum E. 

 The secondary or lesser western minimum falls at. 4 A.M., 11 minimum E. 



The secondary or lesser minimum (the nocturnal elongation westward) 

 falls, more correctly speaking, between 3 and 5 A.M., sometimes nearer 

 the one hour, and sometimes nearer the other." I need scarcely ob- 

 serve that the periods which Enke and myself designate as the eastern 

 minima (the principal and the secondary minimum at 4 A.M.) are 

 named western maxima in the registers of the English and American 

 stations, which were established in 1840, and consequently our Eastern 

 maxima (8 A.M. and 10 P.M.) would, in accordance with the same 

 form of expression, be converted into western minima. In order, there- 

 fore, to give a representation of the horary motion of the needle in its 

 general character and analogy in the northern hemisphere, I will em- 

 ploy the terms adopted by Sabine, beginning with the period of the 

 greatest western elongation, reckoned according to the mean time of the 

 place: 



