404 On the Meridian Instruments of the Dudley Observatory. 
Art. XXXVIII.—On the Meridian Instruments of the Drdley 
Observatory ; by Dr. B. A. GouLp, of Cambridge. 
Proceedings of the American Association for the Advancement of Science at Albany, 
July, i856. 
Mr. Gouup described the meridian-circle and transit-instru- 
ment, now nearly completed for the Dudley Observatory, an 
gave some account of the principles adopted in their construction. 
The meridian instruments now in use in the several observa- 
tories of the world may be classified in two divisions,—which 
may be designated as the German and the English styles,—and 
‘perhaps be justly described, the one as the instrument of the en- 
gineer, the other as that of the artist. For the former the circles 
are large and massive, frequently having a diameter equal to the 
entire focal length of the attached telescope; in the latter they 
are smaljer and slighter. The new transit-circle of Professor 
Airy, at Greenwich, typifies the English style, and this instru- 
ment, with its counterpart at the Cape of G ope, presents 
the merits in the most conspicuous and impressive form. It 1s 
of iron, cast in a single piece; incapable of reversal, for which 
the observation of collimators is substituted; without a striding 
or hanging level, this apparatus being superseded by observa- 
tions of the meridian thread as reflected from the surface of mer- 
cury ; the circle is eight feet in diameter, and read by diverging 
microscopes firmly imbedded in a massive pier; and the pivot- 
forms are investigated by means of a collimating apparatus, of 
which the axis of rotation itself forms a part. 
The instruments of the German school are of an entirely dif 
great diversity of circumstance as is consistent wi 
of the same d : 
