150 Analyses of Books. [Aue. 
axis. By this construction, the telescope is rendered indepen- 
dent of the plane of the circle, and by the length of the axis is 
impelled to move in a truly vertical plane; and by reversing the 
axis, the line of collimation may be perfectly verified. This, 
therefore, renders it also a good instrument for observations in 
right ascension. The second novelty is the application of the 
two small levels or finders, which afford a very great convenience 
when repeating zenith distances; as by this application the 
telescope can be readily placed to those distances each time 
the instrument is reversed without the aid of a divided circle. 
There is also a novelty applied to the lantern which will be found 
extremely convenient. It consists of two plates of brass, having 
a square hole ineach; these plates are moved in contrary direc- 
tions by rack and pinion; and by this contrivance the observer 
is enabled to regulate the light in any proportion that may be 
required. There is also an entirely new application, which will 
be extremely advantageous when taking horizontal angles. This 
is the level which is applied to the principal horizontal circle, and 
which in every respect answers the purpose of a second teles- 
cope, while it is much more convenient, as the observer can 
instantly perceive the least possible motion of the circle without 
the necessity of changing his position; and if it should be 
required to take horizontal angles at night, the advantage will 
be very considerable. There is, lastly, a new appendage which 
will be found very useful when repeating the verticalangles. It 
consists of two arms fitted to the lower end of the centre that 
belongs to the horizontal circle, and has a motion sufliciently 
tight to keep it at the place to which itis set. When the teles- 
cope is presented to the object for observation, one of these arms 
is brought to coincide with a projecting piece in the triangular 
frame, and when the instrument is turned half round by bringing 
the other or epposite arm to coincide with the same projecting 
piece, the object will be again in the field of view of the telescope. 
In the third memoir, Mr. Francis Baily details “ A Method 
of fixing a Transit Instrument exactly in the Meridian.” ‘This 
zealous and distinguished mathematician and astronomer, 
zecommends, that when the transit instrument is placed nearly 
in the plane of the meridian, its accurate adjystment should be 
completed by observing the culmination of any two stars differ- 
ing from each other considerably in declination. By this method, 
the necessity of having a building constructed, so as to command 
an uninterrupted view of the meridian from the northern to the 
southern horizon is avoided, since it may be successfully prac- 
tised with portable instruments placed on the.imner side of a 
window having a range of above 70° in altitude, or on the outer 
side, where they may be directed even to the zenith. ‘ The 
stars which should be chosen for the purpose,” Mr. Baily says, 
«¢ are those which differ at least 50 degrees from each other in 
declination, but the nearer that difference approaches to 90 
