180 THE PHYSICAL GEOGRAPHY OF THE SEA. 



rise up with it, the earth is shpping from under them ; and thus 

 the cloud-ring, though really moving from west to east with the 

 earth, goes relatively slow^er than the earth, and would therefore 

 appear to require a longer time to complete a revolution. 



365. But, unlike the rings of Saturn through the telescope, the 

 outer surfa-ce, or the upper side to us, of this cloud-ring would ap- 

 pear exceedmgly jagged, rough, and uneven. 



366. The rays of the sun, playing upon this peak and then upon 

 that of the upper cloud-surface, melt away one set of elevations 

 and create another set of depressions. The whole stratum is, it 

 may be imagined, in the most turgid state ; it is in continued 

 throes when viewed from above ; the heat which is liberated from 

 below in the process of condensation, the currents of warm air as- 

 cending from the earth, and of cool descending from the sky, all, 

 we may well conceive, tend to keep the upper cloud-surface in a 

 perpetual state of agitation, upheaval, and depression. 



367. Imagine in such a cloud-stratum an electrical discharge to 

 take place ; the report, being caught up by the cloud-ridges above, 

 is passed from peak to peak, and repeated from valley to valley, 

 until the last echo dies away in the mutterings of the distant thun- 

 der. How often do we hear the voice of the loud thunder rum- 

 bling and rolling away above the cloud-surface, like the echo of 

 artillery discharged among the hills ! 



Hence we perceive or infer that the clouds intercept the prog- 

 ress of sound, as well as of light and heat, through the atmosphere, 

 and that this upper surface is often like Alpine regions, which echo 

 back and roll along with rumbling noise the mutterings of the dis- 

 tant thunder. 



368. It is by trains of reasoning like this that we are continu- 

 ally reminded of the interest which attaches to the observations 

 which the mariner is called on to make. There is no expression 

 uttered by Nature which is unworthy of our most attentive consid- 

 eration—for no physical fact is too bald for observation — and mar- 

 iners, by registering in their logs the kind of lightning, whether 

 sheet, forked, or streaked, and the kind of thunder, whether roll- 

 ing, muttering, or sharp, may be furnishing facts which will throw 

 much light on the features and character of the clouds in different 

 latitudes and seasons. Physical facts are the language of Nature, 

 and every expression uttered by her is worthy of our most atten- 

 tive consideration. 



