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Julian: in the morning, light winds with much rain, followed by a very heavy squall with rain, settled into heavy gale with large cumuli, cleared up, blowing very strong from S.S.W. Temperature 60 degs., dew-point 42 degs., difference 18 degs. Rengger, Natur. der Saeugethiere von Paraguay.
By Oltmann's tables the elevation is 18,540 feet. The tension of vapour was .0763, and the dew-point was 5.8 degrees, or 43.5 degrees below the temperature of the air. The difference between the dry cold air of this elevation and that of the heated plains of India, is very great.
The temperature of the dew-point on leaving the earth decreases less rapidly than the temperature of the air; so that the difference between the two temperatures becomes less and less, till the vapour or cloud plane is reached, when they are usually together, and always most nearly approach each other, and that point is usually at about the height of one mile.
These tables of factors have been constructed at the Greenwich Observatory, and are generally used. The Hygrometer, invented by Mr. Daniell, gives the dew-point by inspection. It is an error to suppose that dew falls like rain from the air; it forms on the body which is cooled down below the temperature of the air.
Upon emerging from them at seventeen minutes past one, I tried to take a view of their surface with the camera, but the balloon was ascending too rapidly and spiraling too quickly to allow me to do so. The height of two miles was reached at twenty-one minutes past one. The temperature of the air had fallen to 32 degrees and the dew-point to 26 degrees.
This dew-point, which figures so largely in all well-kept meteorological reports, is the key to many important conditions of the atmosphere, affecting health, vegetation, and climate. It is found that the air at different degrees of heat has different degrees of elasticity, different degrees of tension, and different degrees of capacity to hold vapor.
On one occasion, this drought was so great during the passage of a hot wind, that at night I observed the wet-bulb thermometer to stand 20.5 degrees below the temperature of the air, which was 66 degrees; this indicated a dew-point of 11.5 degrees, or 54.5 degrees below the air, and a saturation-point of 0.146; there being only 0.102 grains of vapour per cubic foot of air, which latter was loaded with dust.
For it will be perceived that an ordinary subtraction of the degrees of temperature on a wet thermometer, which had cooled down by evaporation, from the actual temperature indicated by a dry thermometer, will not give us the dew-point.
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