348 ©. A. Young on a method of determining the Level-error. 
Art. XXXVIL—On anew method of determining the Level-error 
of the axis of a meridian instrument; by C. A. Youne, Ph. ee, 
Professor of Natural Philosophy and Astronomy in Dart 
mouth College. 
[Read at the Troy meeting of the Am. Association for the Advancement of Science ] 
THE inclination of the axis of a meridian instrument to 
horizontal plane has hitherto been measured by three different 
ae eae by the use of the spirit level; by examining with a 
collimating eye-piece the image of the wires as in nadir-point 
beeen the collimation having been gvtigarts determined 
either med reversal of the instrument or by collimators; and 
cote Py observing the transits of stars by reflection from an 
artificial horizon. 
The first of these methods is by far the most used, and with 
portable instruments is sufficiently Sonat Still it requires 
a good deal of time, and, in the case of a large instrument, of 
hard work; and if ‘there are scnetis irregularities upon the 
pivots of the instrument it is a very troublesome operation to 
ascertain and apply the necessary corrections. 
e second and third methods are still La laborious: the 
second gives the level error corresponding to but one single 
position of the telescope, i. e., with the elagons pointing down- 
is therefore liable to a constant error depending 
upon any malformation of the pivots which affects the instru- 
ment in this particular peers the third method can be used 
only when the air is perfect 
he method I have to sic a allows the determination of 
this error bee any further labor than two readings of a 
microscope, in any position of the telescope, and without that 
guosuriortitie climbing enon is involved in the use of the 
striding level or nadir observa 
The annexed diagram iltaatrntee ei arrangement of the 
apparatus, in which, however, no d is paid to the relative 
proportion of parts; the prism and mercurial age being 
eho exaggerated in size for the sake of distinctn 
The axis of the instrument is to be fitted up as a ag 
situated in 1 the other ivot, O. reading ts 
attached to the pier suit provided with an o oe 
ee OE NN ee ee 
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