On the Meridian Instruments of the Dudiey Observatory. 405 
any proposition of the kind,—desiring to take the whole respon- 
sibility, if any,—and were so strenuous that I refrained from 
Pressing the matter. The clear aperture is ninety French lines; 
the focal length, ten English feet. 
Both circles are divided, and capable of rotation round the 
axis, and they are read by means of four microscopes firmly set 
each pier,—horizontal, not converging, although the divi 
silver limb is slightly oblique to prevent the Se 
the lamp from blinding the observer's eye. The piers beihg two 
feet in thickness, and the microscopes read from the outer side, 
t Microscopes are not far from twenty-five inches im length, 
7,2 Circumstance which gives rise to various not unimportan 
disadvantages: but the ingenuity and skill of Mr. Martins have 
surmounted the chief of these, the large amount of expansion 
and contraction of the metal due to changes of temperature, 
with great succes, by leaving the metal tubes free to extend or 
Tecede without hindrance, and without affecting either the 
tance of the lenses or their fixity inthe stone = 
To obviate the chief disadvantages of attaching the micro- 
Some other preparation for excluding moisture, wound around ~ 
fades saa then encased in wood. With these pre- 
Cautions, I am very confident that we are justified in awaiting 
from this more massive construction greater advantages than 
Would be derived from the metallic connection of the micro- 
Scopes, although continually subjected to scratiny by meaus of 
the attached ether-level. : 
