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IX. On the Solar Variations of Magnetic Declination at Bombay. By Charles 
Chambers, Esq., Superintendent of the Colaba Observatory. Communicated by Bal- 
four Stewart, F.B.S. 
Received June 30, — Read December 10, 1868. 
1. The observations discussed in the following pages were made at the Government 
Observatory at Bombay, during the years 1859 to 1865, and constitute a portion of a 
much larger series of observations of which the remaining part is still awaiting reduc- 
tion. With the exception of Sundays, and eight or ten complete days in each year, 
the observations were taken continuously at hourly intervals throughout the period of 
seven years. The observers were carefully trained Brahmins, who were under the im- 
mediate oversight of highly intelligent assistants of the same caste. 
2. The instrument used was made by Grubb of Dublin, and consists of a rectangular 
bar-magnet suspended horizontally, and carrying a divided scale and a lens, by means 
of which its position can be determined from time to time by reading the scale with a 
fixed telescope properly placed. The dimensions of the magnet are 15 inches by 
0-86 inch by 0 # 25 inch, and the broad surface is made to lie horizontal ; the suspension- 
thread was formed of about forty fibres of untwisted silk, and is 35 inches long, being 
protected (but not concealed from view) by a glass tube 1*3 inch in diameter: the bot- 
tom of the glass tube rests upon the top of a cylindrical mahogany box (8 ’5 inches in 
height) which surrounds the magnet, and the top supports a horizontal divided circle 
(the torsion-circle), and a brass cross piece to which the suspension-thread is attached. 
The tube is secured in a vertical position by a mahogany cross bar which has a circular 
hole in its centre that fits over the upper end of the tube, and which is fixed at its ex- 
tremities to two copper pillars whose feet are screwed into the marble basement of the 
instrument ; and the upper aperture of the tube with the attachments is screened from 
the outer air and dust by an inverted hemispherical glass vessel. A sliding-frame which 
carries above a finely divided scale etched upon glass is fixed by a binding-screw near 
the northern extremity of the magnet, and a similar frame carrying a lens, whose focal 
length is about 12‘7 inches, is secured to the magnet at that distance to the southward 
of the glass scale, so that the latter is approximately in the principal focus of the lens. 
The vertical lines which form the scale are equidistant and generally of uniform length ; 
but every fifth division is slightly and every tenth division considerably prolonged, and 
over the tenth divisions numbers are marked in consecutive order, thus allowing a nume- 
rical designation to be given with facility to every point of the scale when viewed by 
the telescope. To permit the scale to be viewed through its lens by a telescope outside 
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