With the advent of the age of geographical discovery it was seen that the climatic features of our globe depend chiefly upon distance from the equator, proximity to the ocean, and height above it. In the tropics especially, the luxuriant vegetation, which diminishes on mountain slopes and higher up gives place to snow, must have been visible proof of the decrease of temperature with altitude, for, as Professor Daniell remarked, mountains are a gigantic registering thermometer having for the freezing-point the line of perpetual snow. The invention of instruments for measuring temperature and barometric pressure made possible the quantitative observations that have supplied the data for deducing the laws governing the atmosphere. The oldest meteorological instrument is, no doubt, the weather or wind-vane, which had its origin before the Christian era. The next oldest is the hygrometer, or instrument for measuring moisture in the air, the form which acts by absorption dating from the middle of the fifteenth century, and the condensation hygrometer being a century younger. Next in chronological order comes the rain-gauge, which appears to have been used by Castelli, a friend of Galileo, in the year 1639. The history of that important instrument, the thermometer, is obscure, but it is certain that Galileo in Padua used an air-thermometer in the latter part of the sixteenth century, which Rey, a French physician, filled with liquid in 1631. This thermometer, as well as other physical instruments, was perfected by members of the Accademia del Cimento at Florence. These instruments are described in _Saggi di Naturali Esperienza_, written in 1666, and translated into Latin and English. The Florentine thermometers had one fixed point, that of freezing water, and contained either spirits or mercury. In 1724 Fahrenheit, in Danzig, fixed three points on the scale of the mercurial thermometer, viz. the cold produced by ice and sal-ammoniac which he called 0∞, freezing water or 32∞, and the heat of the human blood which he assumed to be 96∞. This thermometric scale, having 180∞ between freezing and boiling water, and that of Celsius, with 100∞, are the only ones in scientific use to-day. It is a remarkable fact in the history of thermometers that neither of these thermometers remained in the country where it was invented; thus the thermometer of Fahrenheit, a German, came into use exclusively in England and her colonies, while that of Celsius, a Swede, is now used on the continent of Europe except in Germany, where the thermometer of RÈaumur, a Frenchman, is still in popular use. Of the four fundamental meteorological instruments, the barometer was the last invented. Aristotle had suspected that air had weight, but it was not demonstrated until the middle of the seventeenth century, when the old axiom "that Nature abhors a vacuum" was replaced by the rational explanation, given by Galileo and Torricelli, his pupil, why water