Besides his works on meteorology and physical geography, he published, in 1867, an admirable little volume—"Familiar Lectures on Scientific Subjects." In this he showed that he could write with as much ease and intelligibility for the general public as for the higher order of scientific inquirers. His style in this valuable manual of information has a charm of its own, and entices the reader into the consideration of subjects apparently abstruse. He is earned on from page to page without any great mental effort, and finds himself rapidly mastering difficulties which he had been accustomed to regard as insuperable.
Let us take the first lecture on "Volcanoes and Earthquakes," and obtain a glimpse of Herschel's mode of treatment. He refers to the greater and more permanent agencies which affect the configuration of our planet. Everywhere, he says, and along every coast-line, we see the sea warring against the land, and overcoming it; wearing it and eating it down, and battering it to pieces; grinding those pieces to powder; carrying that powder away, and spreading it out over its own bottom, by the continued effect of the tides and currents. What a scene of continual activity is presented by the chalk-cliffs of Old England! How they are worn, and broken up, and fantastically sculptured by the influence of winds and waters! Precipices cut down to the sea-beach, constantly hammered by the waves, and constantly crumbling; the beach itself made of the flints outstanding after the softer chalk has been ground down and washed away; themselves grinding one another under the same ceaseless discipline—first rounded into pebbles, then worn into sand, and then carried further and further down the slope, to be replaced by fresh ones from the same source. Here the likeness of an old Gothic cathedral, with lofty arch, and shapely pinnacle; there the similitude of a mass of medieval fortifications, with crumbling battlements and shattered towers!
The same thing, the same waste and wear, is going on everywhere, round every coast. The rivers contribute their share to the great work of change. Look at the sand-banks at the mouth of the Thames. What are they, says Sir John Herschel, but the materials of our island carried out to sea by the stream? The Ganges carries away from the soil of India, and delivers into the sea, twice as much solid substance weekly as is contained in the Great Pyramid of Egypt. The Irawaddy sweeps off from Burmah sixty-two cubic feet of earth in every second of time, on an average Sometimes vast amount of earthy materials is transferred from one locality to another by river agency, as is the case in the deltas of the Nile and the Mississippi.
These changes operate silently, continuously, and unperceived by the ordinary observer; but Nature does not limit herself always and everywhere to such peaceful agencies. At times, and in certain places, she acts with startling abruptness and extraordinary violence. Let the volcano and the earthquake attest the immensity of her power. Let the earthquake tell how, within the memory of man, the whole coast-line of Chili, for 100 miles about Valparaiso, with the mighty chain of the Andes, was hoisted at one blow, and in a single night (November 19, 1822), from two to seven feet above its former level, leaving the beach below the old low-water mark high and dry. One of the Andean peaks upheaved on this occasion was the colossal mass of Aconcagua, which overlooks Valparaiso, and measures nearly 24,000 feet in height. On the same occasion, at least 10,000 square miles of country were estimated as having been upheaved; and the upheaval was not confined to the land, but extended far away to sea,—which was proved by the soundings off Valparaiso and along the coast having been found considerably shallower than they were before the shock.
In the year 1819, in an earthquake in India, in the district of Cutch, bordering on the Indus, a tract of country more than fifty miles long and sixteen miles broad was suddenly raised ten feet above its former level. The raised portion still stands up above the unraised, like a long perpendicular rampart, known by the name of Ullah Bund, or God's Wall.
With a similar fertility of illustration, Herschel sets before us the phenomena of volcanic eruptions and their extraordinary effects.
In a district of Mexico, between the two streams of the Cintimba and the San Pedro, on the 28th of September 1789, a whole tract of ground, from three to four miles in extent, surged up like a foam-bubble, or the swell of a wave, to a height of upwards of 500 feet. Flames, lurid and crackling, broke forth over a surface of more than half a square league; and the earth, as if softened by heat, was seen to rise and sink like the rolling tide. Vast chasms opened in the earth, into which the two rivers poured their waters headlong; reappearing afterwards at no great distance from a cluster of hornitos, or small volcanic cones, which sprang out of the mighty mud-torrent that gradually covered the entire plain. Wonderful and awful as were these phenomena, they were surpassed by the sudden opening of a chasm which vomited forth fire, and red-hot stones and ashes, until they accumulated in a range of six large mountain masses,—one of which, now known as the volcano of Jorullo, attains an altitude of 1690 feet above the ancient level.
In like manner Sir John proceeds to describe an eruption of Mount Tomboro, in the island of Sumbawa, the influence of which was felt to a distance of 1000 miles from its centre, in strange tremulous motions of the earth, and in the clash and clang of loud explosions. He says that he had seen it computed that the quantity of ashes and lava ejected in the course of this tremendous eruption would have formed three mountains of the size of Mont Blanc.
As to the nature of the forces which operate to produce this astounding result, Herschel puts forward a theory of singular simplicity and directness.