Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2- Appendix): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

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