ASTKONOMY. 191 



frame M'ork which i.s floated on a small quantity of mercury coutaiiuMl 

 iu a shallow trough. The trough is supported on au upright pillar, 

 and can be turued about a vertical axis, and set with the telescope it 

 carries to any azimuth. If the telescope is chunped at a given altitu<le, 

 the sight line will mark accurately in the heavens a i)arallel of altitude, 

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

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

 means of determining instrumental aud clock corrections, the latitude, 

 or right ascensions aud declinations. The particiUar parallel of alti- 

 tude which has been found most convenient, as it materially siinplilies 

 the formulic of reduction, is the parallel passing through the pole, to 

 ■which the luime '' co-latitude circle" is given. IMi-. Chandler gives the 

 mathematical theory of the new instrument, and illustrates the various 

 formuhe for reducing observations by numerous examples. He then 

 proceeds to examine critically the results of his observations (which 

 are given in detail in the last chapter), aud to describe a number of ex- 

 j)eriments on the stability of the instrumeut. He also suggests several 

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

 a more complete form of the apparatus. 



The almucantar as a field instrument seems to possess many advan- 

 tages over tlie transit and zenith telescope for the determination of 

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

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

 struments; but for the determination of accurate positions of the fixed 

 stars — a class of work for which meridian instruments hav^e hitherto 

 been exclusively employed — it oft'ers a new and independent metiiod, 

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

 The remarkable results that Mr. Chandler has already obtained with 

 his instrument of only about 4 inches aperture certainly justify great 

 confidence in the "almucantar system." 



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

 vard Observatory a horizontal telescope of 12 inchcis a[)erture aud 17 

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

 coude. The tube is placetd east and west, the object-glass at the west- 

 ern end. Before the object-glass is a plane 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 stnall building covering the eyei>iece at 

 the eastern end protects the observer, and 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 intended primarily as a photometer, 

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

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

 reduced by polarizing apparatus to equality' with the image of a star 

 observed iu the principal telescope. The angular apertures of the two 

 telescopes are such that the emergent pencils are coincident. 



