3^1 NEW ASTRONOMICAL INSTRUMENT. 



Description of to the latitude of the place. In order to divide those qua- 

 f^rdyermlning^'**'^*®' "^°^*^ *^^ horary circle, so that 12 o'clock noort 

 the apparent may be exactly opposite to the index B : then since the 

 -zt^Vkl!^ ^ ®star is the equator, and its declination o, when the semi- 

 diurnal arc is VI hours, the zero of the scales of declination 

 will be opposite VI. VI. and as the declination of a star is 

 equal to the colatitude of the place, when its semidiurnal 

 arc is o, or when it just comes to the south point of the 

 horizon, without rising above it, the degree of declination 

 at the other extremity of the quadrant, or opposite XII. 

 XII. will be the same as the colatitude of the place, which 

 in the present case is 39, the latitude of the place being 

 supposed 51° North. The intermediate degrees of declina- 

 tion are then to be laid down from a table of semidiurnal 

 arcs *, by placing the degree of declination opposite to the 

 arc to which it corresponds, thus the 10th degree of south 

 i- declination must stand opposite V*" 13' in the afternoon, 



and VP 47' in the morning, because a declination of ten 

 degrees south gives a semidiurnal arc of V* 13'. When 

 the scales of declination are thus completed, the instru- 

 ^ f ment is ready for shewing the rising and setting of the stars. 



For this purpose move the horary circle till the index B 

 ' points to the time of the star's southing ; thus opposite to 



the stars declination in the scale C, if the declination is 

 south, or in the scale D if it is north, will be found the 

 . time of its rising above the horison ; and the degree of de- 



clination on the scales E and F, according as it is south or 

 tiorth, will point out on the horary circle the time of the J 

 star setting. If the rising of the star is known from ob- 1 

 servation, bring Its declination to the time of its rising on 

 the circle of hours, and the index B will point out the time 

 at which it passed the meridian ; and its declination on the 

 opposite scaie will indicate the time when it descends below 

 the horizon. In the same way, from the time of the star 

 setting, we may determine the time when it rises and comes 

 to the meridian. 



The two exterior circles are added to the astrometer, for 

 the purpose of finding the position of the stars and plan&ts 



* The most accurate table of semidiurnal arcs that I have seen, 

 is published in the Tables de Berlin, Tom. III. p. 233. 



