IZZ COSMOS. 



1847, as the results of many thousand horary observations in 

 the following four turning points, 64 namely, the first mini- 

 mum at 8 A.M.; the first maximum at 2 P.M. ; the second 



54 Airy, Magnetic and Meteorological Observations made at Greenwich 

 (Results, 1845, p. 6, 1846, p. 94, 1847, p. 236). The close correspondence 

 between the earliest results of the nocturnal and diurnal turning hours, 

 and those which were obtained four years later, in the admirable obser- 

 vatories at Greenwich and at Toronto in Canada, is clearly shown by 

 the investigation made by my old friend, Enke, the distinguished direc- 

 tor of the observatory at Berlin, between the corresponding observa- 

 tions 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 Freiberg 

 observations for 1830 (Poggend. Ann. Bd. xix, s. 373). Graphical repre- 

 sentations are preferable to numerical tables for affording a correct 

 insight into this phenomenon. In the former, great irregularities 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 indivi- 

 dual and striking irregularities are mistaken for a true maximum or 

 minimum. The periods seem to fall regularly at the following turning 

 hours : 



The greatest eastern declination falls at 8 A.M. 1 max. E. 



western 1 P.M. 1 min. E. 



The secondary or lesser eastern max. 10 P.M. 11 max. E. 



western min. 4 A.M. 11 min, 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 I 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 expres- 

 sion, be converted into western minima. In order, therefore, to give a 

 representation of the horary motion of the needle in its general charac- 

 ter and analogy in the northern hemisphere, I will employ the terms 

 adopted by Sabine, beginning with the period of the greatest western 

 elongation, reckoned according to the mean time of the place : 



Freiberg. Breslau. Greenwich 



1829. 1836. 1846-47. 



Maximum ,~~ ,-.... 1 P.M. 1 P.M. 2 P.M. 



Minimum 1 A.M. 10 P.M. 12 P.M. 



Maximum 4 A.M. 4 A.M. 4 A.M. 



Minimum ,... SA.M. 8 A.M. 8 A.M. 



