284 THE PHYSICAL GEOGRAPHY OF THE SEA. 



liberated from below in tlie process of condensation, the currents 

 of warm air ascending 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. 

 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, un- 

 til 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 in- 

 fer that the clouds intercept the progress of sound, as well as of 

 light and heat, through the atmosphere, and that this upper sur- 

 face is often like Alpine regions, which echo back and roll along 

 with rumbling noise the mutterings of the distant thunder. 

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

 Exceeding interest allv reminded of the interest which attaches to the 



attached to physical , . t'i.i • • nn . 



research at sea. obscrvatious which the manner is called on to 

 make. There is no expression uttered by nature which is im- 

 worthy of our most attentive consideration — for no physical fact 

 is too bald for study — and mariners, by registering in their logs 

 the kind of lightning, whether sheet, forked, or streaked, and the 

 kind of thunder, whether rolling, muttering, or sharp, may be fur- 

 nishing facts which will throw much light on the features and 

 character of the clouds in different latitudes and seasons. Phys- 

 ical facts are the language of Nature, and every expression utter- 

 ed by her is worthy of our most attentive consideration, for it is 

 the voice of WiSDO^i. 



