The degree of heat at which water passes away into the form of gases is regulated by the pressure of the air on the water and other things about us. At the average level in the United States where people live the pressure of the air on everything is fifteen pounds to the square inch, and at this pressure water boils only after it reaches a temperature of 212° Fahrenheit. As we go up the mountains the pressure becomes less and less as we go up. At the top of Mount Blanc, which is 15,781 feet high, water boils at 185° Fahrenheit. If we took a balloon from the top of the mountain we would come to a height where there was no air pressure at all. What Do We Mean by Fahrenheit? The name Fahrenheit is used to distinguish the kind of scale most commonly used on thermometers in Great Britain and the United States. Gabriel Daniel Fahrenheit, a native of Dantzic, made the first thermometer on which this scale was used, and it is named after him. In this scale for thermometers the space between the freezing point and the boiling point is divided into 180 degrees--the point for freezing being marked 32 degrees and the boiling point 212 degrees. Why Can’t We Swim as Easily in Fresh Water as in Salt Water? Our bodies are heavier than fresh water, i. e., a bulk of fresh water equal to the size of our body would weigh less than our body, so that the first tendency is to sink to the bottom if we find ourselves in fresh water. If man had not learned to swim that is what he would always do, sink to the bottom; but having learned how to keep from sinking, he is able to swim in fresh water. However, we find that an amount of salt water equal to the bulk of a man in size is heavier than an equal amount of fresh water, although such a bulk of ordinary salt sea water will still weigh less than the man. A man will sink in salt water also if he has not learned to swim or float, but he can keep up with less effort in salt water, and also swim in it more easily. In a nutshell, then, the answer to this question is that salt water is heavier than fresh water. You can make salt water so full of salt that it becomes heavier than a man. Great Salt Lake in Utah is so salty that one cannot sink in it for this reason. You could drown yourself in it, of course, by keeping your head under water, but whether in shallow water or deep water you would not sink in Great Salt Lake. Why Do We Say Some Water Is Hard and Other Water Soft?