158 PROCEEDINGS OF THE AMERICAN ACADEMY 



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

 ghiss 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 ftiil 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 Simms of London. The star ). Ursie 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 sti'ong 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 sei-vice 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 expense. 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 

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

 suggestion to use a tree-stump is impractical, on account of the roots : 

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



When circumstances compel the use of a wooden jiost, 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 harmless. 

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

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



