542 LECTURE XLV. 



the precession of the equinoxes, which produces a slight change in the places 

 of the stars, has made it necessary to wait 1 minute 134- seconds for every 

 ten years that have elapsed. Thus, in 1806, if we wait 6~ minutes, the 

 pole star will then be precisely in the meridian, and will serve for the 

 correct adjustment of the instrument. (Plate XXXV. Fig. 507 • • 510.) 



The quadrant in most common use, especially for nautical observations,' 

 was first proposed by Newton, but improved, or perhaps reinvented, by 

 Hadley. Its operation depends on the effect of two mirrors which bring 

 both the objects, of which the angular distance is to be measured, at once 

 into the field of view; and the inclination of the speculums by which this is per- 

 formed serves to determine the angle. The ray proceeding from one of the 

 objects is made to coincide, after two reflections, with the ray coming immedi- 

 ately from the other, and since the inclination of the reflecting surfaces is then 

 half the angular distance of the objects, this inclinatoin is read off on a scale 

 in which every actual degree represents two degrees of angular distance, and 

 is marked accordingly. There is also a kcond fixed speculum, placed a 

 right angles to the moveable one, when in its remotest situation, which then 

 produces a deviation of two right angles in the appareut place of one of the 

 objects, and which enables us, by moving the index, to measure any angle 

 between 180" and 90°. This operation is called the back observation; it is 

 however seldom employed, on account of the difficulty of adjusting the spe- 

 culum for it with accuracy. The reflecting instrument originally invented by 

 Hooke was arranged in a manner somewhat different. (Plate XXXV. Fig. 511.) 



From the meridian altitude of any point, it is easy, when the elevation of 

 the pole is known, to deduce its declination: and its right ascension may be 

 found from the time of its passage over the meridian after that of the equi- 

 noctial point, allowing 15 degrees for each sidereal hour. (Plate XXXV. 

 Fig. 512.) 



In all astronomical observations it is necessary to make proper corrections, 

 according to the rules of optics, for the effects of atmospherical refrac- 

 tion; and also, in observations on the moon more especially, for those of 

 parallax, or the difference of the apparent place of the luminary with 



