APJ-ENDIX. 275 



C F E is an angle at the circumference of a circle, and therefore half 

 C D E, at the centre ; and equal to D E F, or forty-five degrees. 



An object at H being reflected from F along the line FE, will 

 appear in contact with an object at K, which we may here suppose to 

 be the horizon of the sea. But, by looking through the glass F, 

 and bringing an object into contact with the horizon, which is really 

 forty-five degrees above it, the index of the quadrant will be at zero ; 

 and by looking through F, and bringing an object into contact with 

 K, or the horizon, which is reaUy one hundred and thirty-five degrees 

 from it, the index of the quadrant wiU be at ninety degrees. 



The principle being thus shown, it is unnecessary to go farther in 

 this place ; either in explaining how it applies equally well to a quin- 

 tant or sextant, or in describing Mr. Worthington's ingenious method 

 of taking advantage of it, in the sextants he has lately made with 

 power to measure 160°. 



In adjusting or verifying the adjustment of the additional glass, I 

 found that by measiu-ing the angular distance of two fixed stars more 

 than forty degrees apart — ^first carefully by the ordinaiy method, and 

 then using the extra or additional glass — it was practicable to ascer- 

 tain its exact error : the only difficulty I had foreseen in the efficient 

 use of this auxiliary. 



I may add, that the telescope moves parallel to the plane of the 

 instrument, and that there are two sets of numbers referring to one 

 gi-aduation. 



No. 45. 

 On Clouds. 



Clouds may be divided into four classes, called — 

 CiERUs, Stratus, Nimbus, Cumulus. 



Cirrus is the first light cloud that forms in the sky after fine clear 

 weather. It is very Ught and delicate in its appearance ; and gene- 

 rally curling or waving, like feathers, hair, or horses' tails. It may 

 also be called the ' Curl Cloud.' 



Stratus is the shapeless smoke-hke cloud that is most common, 

 and of all sizes : sometimes it is small, and at a distance, like spots of 

 inky or dirty water ; its edges appearing faint or' ill-defined ; some- 

 times it rises in fog-banks from water, or land ; sometimes it over- 

 spreads and hides the sky. Rain does not fall from it. Its exact 



