ACADEMY OF SCIENCES. 



157 



A New and Expeditious Method of Placing the Transit. 



BY T. J. LOWET. 



The most approved methods of finding latitude and time are those with the 

 telescope in the plane of the meridian; and hence this plane is the first object 

 of the practical astronomer's search in the observatory. The methods now 

 understood by the astronomer of getting his instruments in the meridian are 

 all trial methods, each of them finding a meridian only by a series of contin- 

 ued approximations, consuming time and effort proportionate to the skill of 

 the observer, the accuracy of the knowledge of his time and latitude, and the 

 rapidity of the successive appearance of favorable stars on his meridian. 



The method now proposed requires but little practice and less skill to place 

 an instrument in the meridian by one observation only, without any knowl- 

 edge of the latitude or time except to the nearest five or ten minutes. 



The essential idea of this method is to observe two stars of the same, or dif- 

 fering twelve hours in, right ascension, but of different north polar distances, 

 at the instant of their simultaneous passage of our meridian. Now, since our 

 zenith is a point in the plane of our meridian, and since the plane of the de- 

 clination circle of any two stars of exactly the same, or twelve hours different, 

 right ascensions, is by the diurnal revolution of the earth made to coincide 

 successively from east to west with every terrestrial meridian, it is obvious 

 that we have but to (select and) observe two such stars at the instant that 

 they and our zenith are in the same vertical plane and clamp, and we have 

 our instrument fixed in the plane of its meridian. 



To accomplish this simultaneous observation of two such stars with a tran- 

 sit, zenith telescope or theodolite, we attach to the tube of the telescope 

 directly in front of its object-glass a plane mirror, half silvered to admit of 

 direct and reflected vision, with its axis of rotation horizontal, parallel to its 

 plane of reflection and perpendicular to the line of collimation of the tele- 

 scope. Attached to this axis is a small vertical finding circle for setting this 

 mirror at any desired angle with, the collimation line. In form this mirror 

 may be either an elliptical ring with only the quicksilver removed (or the 

 glass also cut away) from its center, or that of the ordinary sextant horizon 

 glass with its silvered half uppermost. This mirror should have its front and 

 back faces perfectly parallel, and be from one-fourth to one-third of an inch 

 thick, so that by having on its front face two fine lines cut at right angles to 

 each other, we can, by making the reflected image of each of these lines coin- 

 cide with its direct image, adjust its plane of reflection perpendicular to the 

 telescope's collimation line, and thus also find the index error of its finding 

 circle. In this adjustment use the collimating eye-piece. 



This method of getting one star of a pair into the telescope field by a single 

 reflection, and the other by direct vision, will work admirably when the stars 

 differ considerably in north polar distances; but when this difference is small 

 and we point on one of the stars direct, it becomes imperative to subject the 



