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Show 113 the beginning of the dew- formation has as yet become apparent on any other object- a fact due to th « lesser porosity and non- hygroscopical qualities, linen, grass, & c, absorbing the aqueous vapor when in commencement of condensation. In this way I frequently compared the actual dew- point with the calculated one, and found in many instances a difference of several degrees, dew making its appearance and beiug visible sooner on the rubber- blanket than can be calculated from the psychrometer- observa-tions. This, however, should not surprise us, if we take into consideration the fact that poor conductors of heat, like rubber, cool off more quickly than the air when left to radiation, and that they have reached the calculated dew- point when the air is yet from 3° to 4° warmer. To what extent the more or less sinking of the nightly temperature depends in these mountains on the absolute humidity of the air may be evident from the following observation : On the afternoon of the 13th of September, a heavy thunder- storm occurred, but toward evening the clouds dispersed. The absolute humidity late in the evening was 8.1 gram per cubic meter, and the next morning the temperature was 45°. On this latter day ( September 14) the air rapidly gained its average normal dryness, and the absolute humidity in the evening was 4.0; therefore the fall of temperature was exceedingly rapid during the night, the thermometer standing the following morning at 23° F. It appeared to me interesting to compare the dryness of the air of New Mexico with that of other countries and climates. In this comparison I found that in a number of instances, in my own observations, the absolu te humidity was lower than any observed in hot and temperate zones. Some of my own observations are given in the annexed table, alongside of those of Humboldt, D'Ahaddie, and Rohlfs, made in the Asiatic and African deserts. The relative humidity, it is true, is in a number of cases in the air of the desert of Sahara still lower- even as low as 6 in one instance; but this is explained by the fact that the temperature of the air was in such cases as high as 122° F., ( 50° C.;) but absolute humidity has never been found as low as in the air of New Mexico. |