4 AMPLITUDE OF THE SOLAR-DIURNAL VARIATION 



the years 1843, 1844, 1845, 1 : 13.3 nearly (the years 1843 and 1845 being incom- 

 plete, and omissions only approximately allowed for). For comparison with the 

 Toronto obserA^ations we have the ratio 1 : 9.4 for the series 1841 to 1848, inclu- 

 sive,^ and 1 : 13.6 for the series 1843, 1844, 1845," both for the limit 3'.6, which 

 was afterwards raised to 5'.0.' It was thought desirable in comparing these results, 

 and especially as the Girard College observations do not extend either way to years 

 of maximum of disturbance, which would otherwise require the enlargement of 

 the limit, to preserve the limit as pointed out by the criterion ; hence a deviation 

 from the normal of 8.0 scale divisions as a convenient number, 3'.64 of arc, has 

 been adopted for the present discussion as constituting a disturbed observation. 

 Previous to July 18, 1840, the declinometer had a different scale, one division being 

 20".7, making the corresponding limit for the first month and a half, 10.6 divisions. 



All observations therefore differing 8.0 scale divisions from the mean monthly 

 value of their respective hour were marked by a pencil line ; a new hourly mean 

 was taken, omitting values so marked, and each observation was again examined 

 with reference to its deviation from this new mean. The process was repeated, 

 when necessary, so that in all cases values differing S'.O or more from the final 

 mean, were excluded. The last mean thus obtained for each observing hour and 

 each month has been called " the normal." The following tables of normals pre- 

 sent the mean monthly declinometer readings for each observing hour, free from 

 all disturbances, deviating either way 3'.64 or more, from the normal position of 

 the magnet for the respective hour, month, and year. The observations having 

 been made at the even Gottingen hours, the local times are 19 i minutes after the 

 even hour.^ The time given in the tables is mean local time, counting from mid- 

 night, or 0" up to 24\ 



Increase in the scale readings, corresponds to a decrease of westerly declination. 

 The value of one division of scale is 0'.453. 



' Observations made at the Magnetical and Meteorological Observatory at Toronto, in Canada, under 

 the superintendence of Colonel Edward Sabine, vol. ii., 1843, 1844, 1845, with abstracts of observations 

 to 1852, inclusive. London, 1853. 



» Phil. Trans. R. S., 1851, art. v. 



' Observations made at the Magnetical and Meteorological Observatory at Toronto, in Canada, under 

 the superintendence of Major-General Edward Sabine, vol. iii., 1846, 1847, 1848, with abstracts of 

 observations to 1855, inclusive. London, 1857. 



* The observations were made at the even Gottingen time, C' 00™, corresponding to 0'' Idh™ of 

 Philadelphia time. 



