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GETHSEMANE

762

GEYSER

rand. In 1810 Gerry was elected governor of Massachusetts, and in 1812 he became vice-president of the United States. As governor he agreed to a bill which changed the districts of the state in such a way as to give a greater number of majorities to his own party. This was the origin of the term gerrymandering, which is still applied to political juggling of any kind.

Gethsemane (geth-sem'a-ne), the scene of the agony of Christ before his crucifixion. It was a small farm or estate at the foot of Mount Olivet, on the east slope of the Kedron valley, rather more than half a mile from the city of Jerusalem. Joined to it was a small garden or orchard, a favorite resort of Christ and his disciples. The place is not now exactly known, but an inclosure with a few old olive-trees is pointed out to travelers as the site of the garden.

Gettysburg, Battle of. After defeating the Union army under General Hooker at Chancellorsville, in May, 1863, General Lee, the Confederate commander, determined on an offensive campaign. Marching rapidly down the Shenandoah valley, he crossed the Potomac at Williamsport on the 24th of June. Pressing forward, he invaded Pennsylvania, passing through Chambersburg toward Harrisburg. Meantime General Hooker followed after Lee, crossed the Potomac at Edward's Ferry, and advanced to Frederick, Md. He wished to withdraw the Union force which was at Harper's Ferry; this the government at Washington would not permit, whereupon he resigned his command, and was succeeded by General George G. Meade. The new commander pushed forward to Gettysburg, near the southern boundary of Pennsylvania, on which point Lee was concentrating his army. Here the hostile armies, each numbering about 80,000 men, joined issue in what proved to be one of the most terribly destructive battles of the war. The struggle began on the ist of July, and raged with fearful carnage for three days. On the afternoon of July 3 the Confederates staked everything in a grand charge on the Union center, but were repulsed with terrible slaughter. The Confederate invasion was at an end. Lee retreated on the 4th, and withdrew his shattered columns across the Potomac. The Confederate loss in the battle was over 30,000; the Union loss was 23,000.

The Union victory at Gettysburg, and the capture of Vicksburg, which occurred at the same time, may be regarded as decisive of the war. The struggle continued for nearly two years, and was marked by terrible battles; but from this time the strength of the Confederacy steadily declined, and the power and authority of the Federal government as steadily advanced.

Geyser, a very remarkable phenomenon which occurs in certain regions of the earth which were formerly volcanic and have re-

tained considerable heat. A typical geyser consists of a crater with a funnel-shaped opening into the earth. From this opening, at fairly regular intervals, a column of hot water and steam is ejected, sometimes to the height of a few feet, say five or six? and sometimes to a height of 150 feet, as in the case of the geyser known as Old Faithful in Yellowstone Park. Until about 1870 Iceland was considered the principal seat of geysers — but we now know that Yellowstone Park contains more geysers than all the rest of the known world.

The geyser was first explained by the chemist Bunsen, of Heidelberg, who went to Iceland in 1847 and made an elaborate stud)r of the Great Geyser. His explanation is as follows: A deep crevasse or well-shaped hole in the volcanic rock is filled with water furnished by springs. This water is heated by the lava which is still hot. The circulation of the water in this long tube not being easy, connective equilibrium is not reached. The water in the lower part of the hole is considerably hotter than that in the upper part; for it is under more pressure than that of the atmosphere. Each horizontal layer of water is hotter than the one just above it.

After the water has reached its boiling-point at the top of the column, anything which disturbs one of these layers, i. e., anything which shifts one of the layers into a position a little higher, will relieve the pressure on this layer. The water of this layer will then be above its boiling-point and will burst into steam. This steam will slightly lift the whole column, thus relieving pressure on all water below the disturbance. Hence the whole thing bursts into steam until it is cooled below the boiling-point. The crater catches considerable of the water which has been thrown up and turns it back into the tube. The remainder is made up by springs which feed the tube. And when the whole has been again heated to boiling-point, the eruption occurs anew.

Some of the geysers in Yellowstone Park throw up mud instead of water. These are known as mud-geysers.

About a geyser, deposits of mineral matter, which was in solution in the ejected water, are often made, the basins being composed of it. The deposition of the mineral matter is due partly to the cooling of the water, partly to evaporation but chiefly to the influence of minute plants (alg&), which live about the geysers, and which have the power of extracting silica, etc. from solution. The beautiful colors of the deposits about geysers are due to the presence of the plants themselves. The deposits often assume fantastic and beautiful forms. See Le Conte's Elements of Geology and Geikie's Geological Studies. For a description of these geysers see any good treatise on geology.

R. D. SALISBURY