Rambaut — Converting Hour-angle and Declination, 8fc. 645 



To find the Azimuth, set NO to the altitude which we have 

 just found. Unclamp U, and bring P up to the point A (fig. 1). 

 Clamp U, and rotate R till Q reads the declination. Slide 8T till 

 it comes into contact with P, and its intersection with the circle M 

 will then give the azimuth. 



Since the section of the pin P is not a point, but a circle of con- 

 siderable radius, in consequence of which 8T is always too far to 

 the right by the length of the radius of P, it is necessary to shift 

 the circle M to the right by the same amount without altering its 

 radius. 



There are some other problems of minor interest which this 

 instrument solves very easily. 



For instance, the time of rising or setting of a heavenly body 

 corrected, if necessary, for refraction, can be very easily obtained. 

 For, since the horizontal refraction may be taken at 36', it is only 

 necessary to set NO to the declination of the body, and the pointer 

 Q to - 36', and the point where ST cuts the circle M, when in 

 contact with P, then gives the hour-angle of rising or setting. 



To find the length of twilight on any given day, find the time 

 of sunset as above. Then, by rotating the piece R till Q reads 

 - 18°, we get the hour-angle of the sun when 18° below the 

 horizon, which gives the end of twilight. 



For stars near the pole, or when a star is approaching the 

 zenith, the rapid increase in the tangents of the declination and 

 altitude will render the determination of the altitude or azimuth 

 difficult, if not impossible. 



SCIEN. fKOC. R.D.S. — VOL. V., PT. VIII. 2 T 



