392 FERGUSON'S LECTURES. 



LECT. prepared, and every thing necessary for fixing it, you 

 ^TV-^ ma y fid tne h ur tolerably exact by a large equinoctial 

 ring-dial, and set your watch to it. And then the dial 

 may be fixed by the watch at your leisure. 



If you would be more exact, take the sun's altitude by 

 a good quadrant, noting the precise time of observation 

 by a clock or watch. Then, compute the time for the 

 altitude observed, (by the rule already described,) and set 

 the watch to agree with that time, according to the sun. 

 A Hadley's quadrant is very convenient for this pur- 

 pose ; for, by it you may take the angle between the 

 sun and his image, reflected from a bason of water : the 

 half of which angle, subtracting the refraction, is the 

 altitude required. This is best done in summer, and 

 the nearer the sun is to the prime vertical (the east or 

 west azimuth) when the observation is made, so much 

 the better. 



Or, in summer, take two equal altitudes of the sun in 

 the same day ; one any time between 7 and 10 o'clock 

 in the morning, the other between 2 and 5 in the after- 

 noon ; noting the moments of these two observations by 

 a clock or watch : and if the watch shews the observa- 

 tions to be at equal distances from noon, it agrees 

 exactly with the sun ; if not, the watch must be cor- 

 rected by half the difference of the forenoon and after- 

 noon intervals ; and then the dial may be set true by 

 the watch. 



Thus, for example, suppose you have taken the sun's 

 altitude when it was 20 minutes past VIII in the morn- 

 ing by the watch ; and found, by observing in the after- 

 noon, that the sun had the same altitude 10 minutes be- 

 fore IV; then it is plain, that the watch was 5 minutes 

 too fast for the sun : for 5 minutes after XII is the middle 

 time between VIII h. 20 m. in the morning, and 

 IV. h. 50 m. in the afternoon ; and therefore, to make 

 the watch agree with the sun, it must be set back five 

 minutes. 



