ASTRONOMY. 191 



frame work whicli is (loated on a siiial! (luantity of iiienuiry coutaiiuHl 

 iu a shallow trough. The trough is supportiMl ou an upright i)illar, 

 aud can be turned about a vertical axis, and set with the teh-seope it 

 carries to any azimuth. If the telescope is clamped at a given altitude, 

 the sight line will mark accurately in the heavens a i)araliel ol" altitude, 

 or "almucantar," and the observation of the time of transits of siais as 

 they rise or fall over this cinde in different azimuths will furnish the 

 means of determining instrumental and clock corrections, the latitude, 

 or right ascensions and declinations. The i)articular parallel of alti- 

 tude which has been found most convenient, as it materially simi)litie,s 

 the formula) of reduction, is the parallel passing through the i)(>le, to 

 Avliich the name "codatitude circle" is given. Mi-. Chandler gives tlio 

 mathematical theory of the new instrument, and illustrates the various 

 iormuhe for reducing observations by numerous examples. He then 

 proceeds to examine critically the results of his observations (which 

 are given iu detail in the last chaptei), and to describe a number of ex- 

 periments on the stability of the instrument. He also suggests several 

 modifications of construction, which he hopes in time to incorporate iu 

 a more complete form of the api)aratus. 



The almucantar as a field instrunu'ut seems to possess many advan- 

 tages over the transit and zenith telescoi)e for the determination of 

 time and of terrestrial latitudes and longitudes, aud Mr. Chandler 

 states that it can be constructed at a much less cost than these in- 

 struments; but for the deternnnation of accurate positions of the tixed 

 stars— a class of work for which meridian instruments have hitherto 

 been exclusively employed — it offers a new and independent method, 

 free from many of the systematic errors inherent in the older system. 

 Th(> remarkable results that Mr. Chandler has already obtained with 

 his instrument of only about 4 inches aperture certainly justify great 

 conhdence in the '^ almucantar system," 



Horizontal telescope. — Professor Pickering has had made for the Har- 

 vard Observatory a horizontal telescope of 12 inches aperture aiul 17 

 feet focal length, possessing some of the conveniences of the equatorial 

 coiule. The tube is placed east and west, the object-glass at the west- 

 ern end. Before the object-glass is a i)lane mirror 18 inches in diame- 

 ter, so mounted that the light of a celestial object not more than one 

 hour on either side of the meridian can be thrown by its means into 

 the field of the telescope. A small l)uilding covering the (!ye-i)iece at 

 the eastern end protects the observer, ami may be heated in winter so 

 that he can work in comfort. The instrument may be used for almost 

 any class of observations, but is iiitended primarily as a i)hotometer, 

 and with this end in view an auxiliary telescope of 5 inches aperture 

 is employed to bring into the field an image of the Pole-star, which is 

 reduced by jmlarizing apparatus to erpiality with the image of a star 

 observed in the principal telescope. The angular apertures of the twq 

 telescopes are such that the emergent iiencils are coincident. 



