TO CORRESPONDENTS.

New Zealand Mail, Issue 1746, 23 August 1905, Page 41

TO CORRESPONDENTS.

Subscriber.—The bet isi off. Frost. —Water deposits on the inside of exposed iron roofs thus. Iron radiates heat faster than air; it there- . fora becomes colder than the surrounding atmosphere. When the difference of temperature becomes considerable the continuously changing layer of air in contact with the iron becomes chilled to “dew point.” It then deposits part of its moisture in minute drops on the iron. If the temperature of the iron falls to 32 degrees the moisture freezes. The same effect may be observed on the inner surface of windows in a warm room when the air outside is cold. The reason of the fall of the water after sunrise, or whenever the iron becomes slightly warmed, is that the minute drops separately adherent to the iron expand with increase of temperature. They come in contact, and in virtue of the force of cohesion run together and form larger drops. These are acted upon by gravity, the pull of which (overcomes their adhesion to the iron, and they fall. This fall may even occur without any rise in the temperature of the iron, if the air is so saturated with moisture as to cause a very copious deposit. This is as “full and clear an explanation,” as space will permit. Look up “Heat” in Gan of s or Deschanel’s Physics'.