248 



KNOWLEDGE 



[Ko\'EMBEB, 1902. 



tii)Uf,'h cuuiaiued luercury, and in it rested a tloat which 

 carried the tclescoi)e. The float was held in ])laci' l>y pins 

 running in slots, which were intended to allow the float to 

 level itself in the mercury, while forbidding motion in 

 azimuth. An axis held the telescope beyond the float 

 clear of the trough, so that it could be clamped at any 



The Case College Almueaiitar. 



altitude. This instrament Mr. Chandler called an 

 " almucantar." The term " almucantar " has almost gone 

 out of use in recent books, but in older ones we find it. 

 For instance, Delambre gives " Alniicantarat , terme arabe; 

 c'est un petit cercle parallele a I'horizon. et dont les 

 poles sont le zeuit et le nadir." It is clear that the optic 

 axis of the instrument described above, when clamped in 

 altitude and moved in azimuth, traces out a horizontal 

 circle upon the sky, and the accuracy with which it does 

 so depends only upon the accuracy with which the floating 

 parts regain their old orientation with respect to the 

 vertical, after the disturbance that occurs when the 

 azimuth is changed. 



In actual use the altitude is always such that the centre 

 of the field passes through the pole, or rather differs from 

 it by a small collimation constant, which is determined 

 afresh each night. Spider lines are placed horizontally at 

 the eye-end, and the co-ordinates of a star are found by 

 two time observations of its transits east and west of the 

 meridian as it is carried across the circle of observation 

 by the earth's rotation. Azimuth is used only for 

 the purposes of setting, and does not appear in the 

 result. 



Mr. Chandler's observations, of the merits of which I 

 shall speak later, were discontinued in 188.5, and found no 

 one to carry them on till the year 1900. In that year, in 

 the month of February, the Durham almucantar was set 

 up, and in March Prof. C. S. Howe erected one at Case 

 College, Cleveland, Ohio. I give figures of each of these 

 instruments, for their differences illustrate well the 

 flexibility of the design. The Durham almucantar and 

 that of Case College are each of six inches aperture, that 

 is to say, of as great power as any transit circles, excepting 

 one or two of the largest. In both the telescope axis is 

 horizontal, the view of the skies being gained by the 

 intervention of a ])lane mirror ; in both the trough and 

 float ai'e made of cast iron, and allow about the same 

 margin of mercury at the sides. Beyond this we might 

 almost say that if two instruments had been designed by 

 the same man to experiment ujion the difl'erent ways of 

 doing the same thing, they could not have served that 



purpose better, and we may expect interesting results as 

 to which form is preferable from the comparison of the 

 Durham results with those of Prof. Howe. In the Durham 

 instrument the mirror is behind the object-glass, and 

 different altitudes are securt-il by rotating the telescojie 

 tube in Y's ; in the other the mirror is exposed, and is 

 itself in slight degree adjustable. In the former tht; 

 trough is rectangular, with sides about 2 feet by 3 feet and 

 about 2 inches deep, and forms part of a large table which 

 turns about a vertical axis, carrying the floating parts with 

 it, which are fixed relatively to it by means of brackets 

 which bear against agate plates on the outside of the 

 trough. In the latter the trough is ring-shaped, about 

 5 feet external diameter and 4| inches deep, and is fixed 

 to the piers, the telescope and float turning in the trough, 

 and guided in their motions by a pin at the centre working 

 in oil. In the former the clamp and slow motion work upon 

 the edge of the rotating table; in the latter they work upon 

 an arm projecting from below the centre of the telescope, 

 and designed so as to limit only the azimuth, a,ud not at 

 all the orientation of the floating parts with respect to 

 the vertical. In both telescopes ttiere is little doubt that 

 this essential and that of rapid subsidence of oscillations 

 after setting are fully secured. Without criticising Prof. 

 Howe's design, I have found the rotating table in my 

 own of the utmost convenience. I have now built up 

 upon it a wind screen, which is designed to cover in 

 entirely the floating parts. It will be seen in the plate 

 that a long arm runs out from it, carrying an eye-guard ; 

 since the photograph was taken this has been removed, 

 and the arm now carries the eye-piece itself with a sliding 

 arrangement, so that the eye-piece is actually detached 

 from the telescope. As to the success of this arrange- 

 ment I was not very confident in advance, but it was 

 necessary to try it, because oblique transits carry the star 

 across the wires at the limits of the field. Happily it 

 turned out a thorough success. 



Now making a comparison of transit circle and almu- 

 cantar in respect to their observations, there can be no 

 doubt that although the almucantar is disturbed by each 

 fresh setting, when the oscillations have subsided it 



Tin- Case 



,Uii 



resumes its old orientation to the vertical within perfectly 

 insensible limits. The effect of this is the same as if in 

 the transit circle we could write the errors of azimuth and 

 level permanently at zero. 



The corrections for refraction are immensely reduced, 

 since observations are all made at the same zenith distance. 



