10 Rev. S* J. Perry on Magnetic Observations, etc. [Jan. 10, 



At the Observatories of the Cape, of Colaba at Bombay, and of Rome, it 

 was only necessary to enter the approximate local mean time under 

 the heading, " Chronometer," as the declination at these stations was 

 measured from the standard marks. The bearing of these magnetic 

 marks is respectively 0° 31' 51", 0° 26' 26", and 21° 26' 54" E. of the 

 Astronomical North Point. At the remaining stations the sun's 

 azimuth was computed for the hour angle of the sun given by the 

 chronometer, except at the college of Bombay, where it was found 

 more convenient to observe the stars Aldebaran and Capella, instead 

 of the sun. The observations marked with an asterisk were taken 

 with the needle of the Cape Observatory. The computed altitudes of 

 the sun for the three observations at Moncalieri, were 63° 3' 33" "60, 

 63° 48' 57"-69, and 64° 29' 17" '78. , This "difference of altitude being so 

 slight makes the correction for the perpendicularity of the mirror 

 much less reliable than it might otherwise have been. 



The declination observations at the Cape of Good Hope date back 

 as far as 1600, and the mean yearly amount of the secular variation 

 for the first 240 years was found to be + 7'' 66 W. It then dimi- 

 nished, and was + 0'"49 from 1841 to 1846, +1''41 from 1843 to 

 1848, and 2' - 16 from 1846 to 1850. The renewed acceleration in the 

 annual variation seems to have lasted but for a short period, as a 

 comparison of the mean value for 1874, with 29° 14''62, the mean for 

 1848, gives only l'"68 as the annual change. 



From observations of this magnetic element at Bombay in 1845, 

 1656, 1867, and 1875, we notice a gradual easterly motion of the 

 needle, it being at the above dates 0° 13' W., 0° 19' B., 0° 42' E., and 

 0° 56' E., shewing a decreasing yearly change of 2'"91, 2' f 09, and l'"75, 

 The amount of the mean annual variation given by the series of 

 monthly observations at the Colaba Observatory from 1867 to 1873 is 

 l'"77, which may be considered identical with the above. 



At Aden the heat of the sun was so great that it was imprudent to 

 remain long near the theodolite whilst taking the solar observation, 

 the declination was therefore obtained under very disadvantageous 

 circumstances, and consequently less weight can be attached to the 

 result. This is apparent also from the abnormal increase in the secular 

 acceleration. Thus the declination which was 5° 2' W. in 1834, 

 became 4° 15' W. in 1857, the annual variation being, therefore, 

 — 2'-04; whilst in 1875 the angle observed was 2° 19' 39" W. } which 

 increases the yearly change to — 6' '41. 



The declination in 1875 at Malta, combined with the value 

 15° 20' WV, obtained in 1834, gives a yearly change of — 4'*76; and 

 the angle 13° 49' measured at Rome in 1852-5, along with the value 

 found in 1875, makes the annual variation — 5''95. For the remain- 

 ing stations I am unacquainted with any published values of this 

 element of the earth's magnetic force, but the careful series of 



