=Altitude Measured by Change in Boiling Point of Water.= The boiling point of water at sea level and ordinary air pressure is 212°. If the pressure of the atmosphere were increased to about thirty pounds, instead of about fifteen to the square inch it would be necessary to raise water to 250° before boiling would begin. The changes of air pressure due to the passage of the severe storms of winter may cause the boiling point of water to vary from 207° to 215°. This knowledge may be useful in measuring the heights of mountains, although the method does not give close results. The decrease of pressure with altitude lowers the boiling point, the amount being approximately one degree for each 555 feet of ascent. The best results may be secured by having a person at the base of the mountain, where the elevation above sea level is known, determine the boiling point at the same time that a person on the mountain top does. The thermometers should be read closely to the fraction of a degree. With the barometer at its normal height of thirty inches, air at 60° will instantly rise to the phenomenal temperature of 175.50 if it be confined and its pressure doubled, and it will diminish to one half of its former volume. But if its pressure be diminished one half, its volume will expand to double its original size and its temperature will fall from 60° to 2.4°. From these facts the reader would naturally expect to find low pressure of the atmosphere accompanying cold waves and high pressure to be coincident with warm conditions, which is exactly the reverse of what actually occurs in the free air of nature. This apparent contradiction will be made plain in the treatment of cold waves, page 124. A temperature of -459° on the Fahrenheit scale and -273.1° on the Centigrade represents what is called absolute zero. It is supposed to be the temperature at which there is no motion of the molecules of matter. Bodies or planets without atmospheres have temperatures approaching absolute zero, for there is no protecting envelope to absorb heat or to prevent the instant radiation into space of that which impinges upon the body. Our moon is an illustration, and notwithstanding the fierce beating upon its surface of the solar energy it remains incased in the intense cold of space.