Page:Popular Science Monthly Volume 22.djvu/661

This page has been proofread, but needs to be validated.

ing a region on the east side of the State, from Jacksonville as far south as Palatka. For a country lying on a parallel with the Canaries, off the coast of Africa, they indicate a climate which in temperature approaches that of Malaga, Malta, and Algiers, but does not nearly equal them in evenness and unchangeableness, though, in point of clear, sunshiny weather, far superior.

However, from the facts given, we are unable to fix accurately the position of the Florida climate. We neither know the atmospheric humidity nor the electrical potential; and, as yet, the phenomena of nature have not been interrogated for an answer.

A cardinal question is whether the climate be moist or dry, bracing or relaxing.

It may not be amiss to note here an error committed by a professional gentleman of Jacksonville.[1] In a pamphlet on the climatology of Florida, he gives tables of the mean relative humidity of Jacksonville and other stations in Florida, and attempts to prove therefrom, by comparison with similar observations in Northern States, that the atmosphere of Florida is dry, much drier than that of Minnesota, Mount Washington (New Hampshire), Alpina (Michigan), Omaha (Nebraska), and other Northern localities. It should be remembered, however, that there is a wide difference between relative humidity and absolute humidity, and their relation is frequently diametrically opposite. Relative humidity does not indicate the amount of vapor present in the air per cubic foot, but only the tendency to deposit it in a wet state on a surface but little lower in temperature than the surrounding atmosphere. Absolute humidity, on the contrary, is the actual amount of vapor present in each cubic foot of air. To illustrate, suppose the cubic foot of air to be a hollow cubic vessel of tin. Absolute humidity is the actual amount of watery vapor contained in that tin vessel. Relative humidity is the tendency of that vapor, be it great or small in quantity, to leak out of the vessel and show itself on the outside in the form of mist or dew. Sir John Herschel says, "As a general meteorological fact, there is not merely a want of accordance, but an actual opposition between both the diurnal and annual progress of the 'degree of humidity' or 'relative humidity' of the air and the 'tension of vapor' as indicated by hygrometric observation, a seeming paradox, but one very easily explained."[2] He then shows how the relative humidity is greatest just before sunrise of each day, and the vapor tension or absolute humidity is least; that as the day advances the relative humidity diminishes and the absolute humidity increases, till the maximum temperature of the day is reached, when absolute humidity is greatest and relative humidity is least. It is also well to know, in considering this question, that air at a temperature of 60° Fahr. is capable of containing double the quantity of vapor by weight

  1. Dr. J. C. Kenworthy.
  2. "Meteorology," by Sir John F. W. Herschel, Edinburgh, 1861, p. 193.