thirty-six degrees, and full ten inches of water must be condensed, setting free heat which would boil four and a half feet of ice water. So large a condensation as this never takes place in twelve hours, partly because the full amount of vapor which might be formed is never actually produced, and partly because the condensation of but a small part of this vapor would check the fall of temperature and prevent farther condensation. The supposition that I have made shows the possibilities of this method of moderating extremes of heat and cold. Were it not for these processes, our days would be much warmer and our nights much cooler than they now are. By the formation of vapor the excess of heat during the day is stored up in a latent form; that is, it is used, not as heat, but as force, and is employed in bringing the atoms of water into new relationship; during the night the vapor returns to its former state as water, and the heat-force again becomes sensible heat. Thus the day is cooled and the night made warmer. "Ansel, have you ever heard the 'dew point' spoken of?" "Yes, sir, I have." "Do you know what is meant by it?" "That point or degree of temperature at which dew begins to be formed." "Upon what does the dew point depend?" "Upon the amount of vapor in the air." "That is right, Ansel. If at any time the full possible amount of vapor should exist, any diminution of the temperature must, of course, cause dew to be deposited. Do you know, Ansel, how to ascertain the dew point at any time?" "No, sir, I do not." "There is a beautiful instrument known as Daniell's Hygrometer which shows the dew point as a thermometer shows the temperature. But any one can easily determine the dew point without a special instrument for that purpose. Pour warm water into a glass pitcher or goblet whose outer surface has been wiped perfectly dry, and polished. Into this set a common thermometer. Cool down this warm water by dropping into it small pieces of ice, and notice carefully when the polished glass begins to be dimmed as if it had been breathed upon. When that begins to take place the thermometer will show the dew point. In this manner we can determine the amount of vapor in the air, and by estimating the probable temperature of the night judge of the probability that dew will fall." "I have noticed some things," said Peter, "about the formation of dew which I do not understand, and I wish very much to ask about them." "I should be glad to hear your questions, and will answer them if I can." "I have noticed that dew falls on clear nights, but not very often on cloudy nights. I don't see why that is so." "Have you ever noticed whether cloudy nights or clear nights are the warmer?" "Cloudy nights are commonly warmer, I think, but I never could see the reason for that, either." "Can you tell why a newspaper spread over a tomato vine keeps the frost from the vine?" "Because the frost comes upon the paper instead of the vine, of course."