158 PROCEEDINGS OF THE AMERICAN ACADEMY 



think a focal length of 14. to 16 inches, and aperture of the object- 

 jrlass of 1| inches, are the best dimensions. The frame should be 

 rather light, but solidly put together; the setting circle plainly divided ; 

 the striding level delicate ("2" to one division) ; and, above all, the ap- 

 paratus for illuminating the field as perfect as can be made. 



Such an instrument ought to show a star of the sixth magnitude with 

 full illumination, if the observer's eye be accustomed to not too bright 

 a light, and especially if he use Mr. Rogers's rulings on glass in 

 place of spider-lines : indeed, he will hardly fail to do so, for other 

 reasons. The instrument of this size with which I am acquainted, 

 belonging to the Canadian government, was planned as an alt- 

 azimuth by Lindsay Russell, Esq., Deputy Surveyor-General, and 

 made by Simnis of London. The star X Ursa3 minoris of the 6.7 mag- 

 nitude could be readily observed with it. It is not too heavy, with all 

 the attachments, to be carried on a strong man's back ; nor too large 

 to accompany the observer in a sleeping-car. A somewhat larger tran- 

 sit, by Temple of Boston, did excellent service on the south boundary 

 of Wyoming. This has a two-inch aperture, a pretty long focal dis- 

 tance, but a short axis and a light frame. It looks ill-proportioned, 

 owing to the length of its telescope ; but has a very excellent object- 

 glass. Its greatest fault is instability in collimation ; the telescope 

 tube seems weakly put together ; and the mounting, as I used it, was 

 unstable too, probably because it was fastened to a plank on a wooden 

 post. 



To mount such an instrument away from civilization requires a 

 good deal of trouble and e:?pense. Brick is, of course, the best material 

 for the foundation, but cannot always be obtained ; and, at one of my 

 stations, the only two brick-masons in town were intoxicated, and the 

 pier was built by a civil engineer who accompanied me, with a ser- 

 geant of the United-States Engineers to mix the mortar. At Duluth 

 the ground itself furnished rough stone in place. 



At Santa Fe an unfinished and abandoned State-house furnished a 

 pier of cut stone. At Fort Union, the sun-dial of the fort, removing 

 the gnomon, was an excellent pillar for the instrument. Chauvenet's 

 suggestion to use a tree-stump is impi'actical, on account of the roots : 

 the instrument is kept in constant tremor by persons walking about. 



When circumstances conij)el the use of a wooden post, great care 

 must be taken to shield it from the sun, and the observations must be 

 so distributed that the changes in azimuth and level are lu>rmless. 

 The level requires constant watching, but ought not to be changed 

 during a group of stars, lest the azimuth be disturbed too. To climi- 



