1866.] 289 [Davidson. 



uted to undetermined errors of inclination arising from unknown 

 inequalities and irregularities of the pivots. 



In 1859, at the Coast Survey Station '• Sulphur Peak," California 

 (3471 feet above the ocean), the transit telescope exhibited such con- 

 stant irregularity of clock errors with certain stars beyond the use of 

 the level, that no confidence could be placed in the results, and I then 

 devised this level for overcoming the difficulty. 



The general idea of the frame of this level is that of three sides 

 of a rectangle, lying in a horizontal plane, having the two shorter 

 .sides resting at right angles upon the transit axis pivots, with the 

 long side carrying the level-tube parallel to the transit axis, and far 

 enough in front of the telescope not to be touched by the transit 

 axis. This, of course, presupposes the parts of the shorter sides or 

 arms beyond the pivots to be heavy enough to counterpoise the parts 

 in front. Upon the long side or cylinder in front the level-tube is 

 secured with all the usual means of adjustment in the strider. 



This level will be heavier than the ordinary level, which weighs 

 eight pounds in the forty-two inch portable transits of the Coast Sur- 

 vey. Composed of the same material, it is estimated to weigh about 

 fifteen pounds, but if made of aluminum-bronze, the weight will be 

 slightly reduced with the parts of the same proportions. On account 

 of the superior qualities of the bronze, the parts may be made 

 lighter. 



To reduce the chances of injury to the pivots by this level, as well 

 as by the strider, I use a long, thin, flat spring, secured to the outer 

 or inner side of each short arm, in a horizontal plane with the top of 

 the pivot when in po.sition, with one end secured and the other con- 

 trolled by a set screw. When the level is placed on the pivots, the 

 middle of the length of each spring rests upon the outer or inner part 

 of its respective pivot, not bearing upon the supporting Y's ; relieves 

 the wearing parts of the pivots from the full weight of the level Y's, 

 and protects them, in a measure, from the full force of all sharp 

 blows. If the level were allowed to remain in position upon the 

 pivots whilst the telescope was revolved for different altitudes, the 

 increased weight would wear the pivots seriously in the supporting 

 Y's, as the portable transits have no counterpoises. To obviate this, 

 and also the ordinary wear of the pivots, a similar application of the 

 spring could be introduced below the pivots, to relieve their pressure 

 from the supporting Y's. This I proposed to the Superintendent of 

 the Coast Survey, about ten years ago. With the twenty-five pounds 

 weight of the present transit telescopes, the pivots rapidly and badly 



