ASTRONOMICAL INSTRUMENTS. 

 Fig. 141. 



139 



Universal Joint Dial and Compass. (Fig. 141, as above.) 

 First set up the arc, which moves upon a joint, and con- 

 tains 90 of latitude ; then raise the plane of the dial that 

 has the hours engraved upon it, to the number of degrees 

 required for the latitude of your place. 



Example. For New York 40 40' ; suffer the compass 

 needle to traverse freely upon its centre ; then turn the 

 compass till the north pole of the needle (which is denoted 

 by a cross wire near the extremity) settles at the north point 

 on the face of the compass, then level the whole by means 

 of the three feet or adjusting screws, and the spirit levels 

 contained within the compass. 



The pole, or gnomon, must be raised perpendicular to the 

 plane of the dial, from the 10th of March to the llth of 

 September, between which times your hour-shadow will fall 

 on the plane of the dial ; and from the llth of September 

 to the 10th of March, the pole or gnomon must be reversed, 

 the shadow of which will then fall on the inside of the ring 

 which contains the corresponding hours with those on the 

 lane. 



