“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.”