The origin of the glacial cirque is entirely different and is said by W.D. Johnson (Journal of Geology, xii. No. 7, 1904) to be due to basal sapping and erosion under the bergschrund of the glacier. In this he is supported by G.K. Gilbert in the same journal, who produces some remarkable examples from the Sierra Nevada in California, where the mountain fragments have been left behind “like a sheet of dough upon a board after the biscuit tin has done its work”; so that above the head of the glaciers “the rock detail is rugged and splintered but its general effect is that of a great symmetrical arc.” Descending one of the bergschrunds of Mt. Lyell to a depth of 150 ft., Johnson found a rock floor cumbered with ice and blocks of rock and the rock face a literally vertical cliff “much riven, its fracture planes outlining sharp angular masses in all stages of displacement and dislodgment.” Judging from these facts, he interprets the deep valleys with cirques at their head in formerly glaciated regions where at the head there is a “reversed grade” of slope, as due to ice-erosion at valley-heads where scour is impossible at the sides of the mountain but strongest under the glacier head where the ice is deepest. The opponents of ice-erosion nevertheless recognize the very frequent occurrence of glacial cirques often containing small lakes such as that under Cader Idris in Wales, or at the head of Little Timber Creek, Montana, and numerous examples in Alpine districts.
CIRTA (mod. Constantine, q.v.), an ancient city of Numidia, in Africa, in the country of the Massyli. It was regarded by the Romans as the strongest position in Numidia, and was made by them the converging point of all their great military roads in that country. By the early emperors it was allowed to fall into decay, but was afterwards restored by Constantine, from whom it took its modern name.
CISSEY, ERNEST LOUIS OCTAVE COURTOT DE (1810-1882), French general, was born at Paris on the 23rd of September 1810, and after passing through St Cyr, entered the army in 1832, becoming captain in 1839. He saw active service in Algeria, and became chef d’escadron in 1849 and lieutenant-colonel in 1850. He took part as a colonel in the Crimean War, and after the battle of Inkerman received the rank of general of brigade. In 1863 he was promoted general of division. When the Franco-German War broke out in 1870, de Cissey was given a divisional command in the Army of the Rhine, and he was included in the surrender of Bazaine’s army at Metz. He was released from captivity only at the end of the war, and on his return was at once appointed by the Versailles government to a command in the army engaged in the suppression of the Commune, a task in the execution of which he displayed great rigour. From July 1871 de Cissey sat as a deputy, and he had already become minister of war. He occupied this post several times during the critical period of the reorganization of the French army. In 1880, whilst holding the command of the XI. corps at Nantes, he was accused of having relations with a certain Baroness Kaula, who was said to be a spy in the pay of Germany, and he was in consequence relieved from duty. An inquiry subsequently held resulted in de Cissey’s favour (1881). He died on the 15th of June 1882 at Paris.
CISSOID (from the Gr. κισσός, ivy, and εἰδος, form), a curve invented by the Greek mathematician Diocles about 180 B.C., for the purpose of constructing two mean proportionals between two given lines, and in order to solve the problem of duplicating the cube. It was further investigated by John Wallis, Christiaan Huygens (who determined the length of any arc in 1657), and Pierre de Fermat (who evaluated the area between the curve and its asymptote in 1661). It is constructed in the following manner. Let APB be a semicircle, BT the tangent at B, and APT a line cutting the circle in P and BT at T; take
a point Q on AT so that AQ always equals PT; then the locus of Q is the cissoid. Sir Isaac Newton devised the following mechanical construction. Take a rod LMN bent at right angles at M, such that MN = AB; let the leg LM always pass through a fixed point O on AB produced such that OA = CA, where C is the middle point of AB, and cause N to travel along the line perpendicular to AB at C; then the midpoint of MN traces the cissoid. The curve is symmetrical about the axis of x, and consists of two infinite branches asymptotic to the line BT and forming a cusp at the origin. The cartesian equation, when A is the origin and AB = 2a, is y²(2a - x) = x³; the polar equation is r = 2a sin θ tan θ. The cissoid is the first positive pedal of the parabola y² + 8ax = 0 for the vertex, and the inverse of the parabola y² = 8ax, the vertex being the centre of inversion, and the semi-latus rectum the constant of inversion. The area between the curve and its asymptote is 3πa², i.e. three times the area of the generating circle.
The term cissoid has been given in modern times to curves generated in similar manner from other figures than the circle, and the form described above is distinguished as the cissoid of Diocles.