Page:The New International Encyclopædia 1st ed. v. 11.djvu/24

ISOTHERMAL. ISOTHERMAL, i's6-thfr'i»«l (from Gk. taoi, isos. eijiial -r dipinT). thcrmf, hoiit). In pliysics, an isotlieriiuil is a line drawn on any diagram 80 as to represent the suecessive values of the properties of a body as it undergoes certain changes, the temperature being kept constant. Thus, if the properties of the body which are to be obsened and noted are its pressure and volume, the isothermal curves arc drawn on a diagram having pressure and volume as ordinates and absciss:r. The isothennals of a typical sub- stance, carbonic-acid gas, COj, in the form of

vapor and liquid, are given in the accompanying figure. Considering any of the lower curves — i.e. the isothermals for temjx'ratures less than 31° C. — it is seen to consist of three parts; the nearly vertical portion is the isothermal for the liquid; the horizontal portion, for the process of evaporation of the liquid; the curve at the right, which is nearly an hyperbola, is for the vapor after all the liquid has evaporated. The isother- mals for temperatures higher than 31° C. do not have any horizontal portion, showing that as the gaseous substance is compressed at these temperatures it never condenses and becomes a liquid — if it did condense, the curve would be- come horizontal, because when the temperature is kept constant the pressure of the condensing vapor does not change. (See Heat.) There- fore, if the gas is to be liquefied, it must be at a temperature lower than 31° C. — i.e. lower than the temperatire corresponding to the first isothermal which dees not have a horizontal por- tion. This temperature is called the 'critical' tempcrnturc.

ISOTHERMAL LINES, or IsoTnERMs. In meteornlng^-, lines that connect places having the same temperature for a day, month, or for any given interval of time. Systems of isothermal lines for daily maps, as well as for monthly and annual means, present in a very graphic manner the prevailing temperature conditions, and are therefore of universal use in meteorologj'. They seem to have been first used by Alexander von Humboldt in 1817; but lines of equal departure from normal values were independently used by Brands in his studies of the daily weather map and the storms of Europe. Isothermal lines co- incide only accidentally, if at all, with small circles of latitude, owing to the great inlluence of the distribution of land and water on the temperature of the air. The temperatures ob- served at the respective stations must be cor- rected or reduced to the standard level surface of the globe, in order that they may be compar- able with each other. This reduction intro- duces discrei)ancies. but there are many reasons for retaining it. The rate of reduction adopted by llann and other climatologists is 0.50" C. per 100 meters, or 1° F. for ;i70 feet. If a chart has btcn constructed using this rate of reduction, and one should desire to know the temperature at the top of a mountain, whose altitude is given, we have but to read from the chart the tempera- ture at sea-level, apply the reduction to the given altitude, and get at once appro.vimately the temperature of the summit. The accuracy of this method is, however, limited by the fact that the rate of diminuti(m of temperature varies so much that the adoption of an average rate is liable to lead us astray. Charts of isotherms for each month of the year and for the whole globe have been constructed, first by Huchan, and afterwards by the United States Weather Bureau, and were published in full in 1000 in ele- gant style in Bartholomew's I'htjsical Atlas. Iso- thermal lines can be calculated and drawn for the upper levels of the atmosphere by using observa- tions on mountain-tops and in balloons and kites. Lines of this character were first drawn by Hcrgc- scll for Europe for certain days in ISDS-IDOO, on which sullicicnt data were collected by means of balloons and kites. These lines show that large variations in temperature occur at the highest attainable levels, and that the atmosphere in general is a mixture of masses of descending cold air and ascending warm air. See charts under Temperaturk. Tkrrestri.vi^; and Distri- lUTiox OF Animals: al-so articles Climate; Distribution of Animals; Meteobology.

ISOTTARD, e'zrrr.'iir', Niccol6, known as XiccoLo i)E Malte in France (1775-1818). An Italian composer and musician, born in Malta. He was educated for the navy, and afterwards entered commercial life: but meanwhile he stud- ied the piano under Pin in Paris, and harmony under Vella and Aznpanli in Malta. Later, while in Italy, he received further instruction from Amendola, Sala. and Guglielmi. In 1795 he produced his first opera L'aviiso ai mnritnti. The reputation gained by several works of the kind obtained for him the position of chapel- master to the Order of Saint .lohn of .lerusalem in Malta. In 1708 he went to Paris, and met there Kodolphe Kreutzcr, who collaborated with liim in two of his operas. One of his first suc- cesses was Michel- Awie (1802). He also be- cam.e known as a pianist. He wrote for the Opf-ra Comique until Boieldieu's grow-ing popu- larity, and the election in 1817 of that composer to the Academy, an honor which Isouard covet- ed, caused him to abandon himself to dissipa- tion. The next year he died of consumption. He had much dramatic tact, his music is always