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From the 10th of October to the 3rd of November, at nightfall, a reddish vapour arose in the horizon, and covered, in a few minutes, with a veil more or less thick, the azure vault of the sky. Saussure's hygrometer, far from indicating greater humidity, often went back from 90 to 83 degrees. The heat of the day was from 28 to 32 degrees, which for this part of the torrid zone is very considerable.
"To determine the temperature of the dew point by Daniell's dew point hygrometer and Regnault's condensing hygrometer, and by the use of the dry and wet bulb thermometers, as ordinarily used, and their use when under the influence of the aspirator, causing considerable volumes of air to pass over both their bulbs, at different elevations, as high as possible, but particularly up to heights where man may be resident, or where troops may be located, as in the high table-lands and plains of India; with the view of ascertaining what confidence may be placed in the use of the dry and wet bulb thermometers at those elevations, by comparison with Daniell's and Regnault's hygrometers; and also to compare the results as found by the two hygrometers together.
But here again radiation comes in to complicate matters; for clouds may check the formation of dew. It may safely be said, however, that other conditions being favourable, a fast run is likely to occur at any time of day should the dew point be reached. Thus the hygrometer is worthy to be studied on a hunting morning. In May there is a good deal of weed-cutting to be done on a trout stream.
I remarked, in the village of Caracas, that the wind of Petare sometimes contributes singularly to give a pale tint to the celestial vault. The wind ceased at three in the afternoon. Thermometer 21; hygrometer 39.3; cyanometer 16 degrees. The atmosphere was then remarkably transparent, without clouds, and of extraordinary dryness.
He climbed the Buet as early as 1770, and wrote an account of his adventures on its summit and its slopes which has the true charm of Arcadian simplicity. He came to England, was appointed reader to Queen Charlotte, and lived in the enjoyment of that office, and in the gratifying knowledge that Her Majesty kept his presentation hygrometer in her private apartments, to the venerable age of ninety.
It crawls forward in all weathers, like Richard Edgeworth's hygrometer. It does not stand at the boundary of our ignorance, it seems to me, but is one of the will-o'-the-wisps of its undisputed central domain of bog and quicksand. Yet I should not have devoted so many words to it, did I not recognize the light it has thrown on human actions by its study of congenital organic tendencies.
As soon as this invisible air was proved to be matter, the whole force of scientific inquiry was directed toward it. The thermometer, by which its heat or cold could be measured the hygrometer, which weighed, literally by a hair, its moisture or dryness were the results of the research of comparatively a few years. Somewhat later came the curious instrument which measures its velocity.
When a height of about 20,000 feet had been reached thunder was heard twice over, coming from below, though no clouds could be seen. A height of 4,000 feet more was attained, and shortly after this Mr. Glaisher speaks of feeling unwell. It was difficult to obtain a deposit of dew on the hygrometer, and the working of the aspirator became troublesome.
In the port of Guayaquil, as everywhere else in the low regions of the torrid zone, the weather grows cool only after storms of rain: and I have observed that when the thermometer sinks to 23.8 degrees, De Luc's hygrometer keeps up to fifty and fifty-two degrees; it is, on the contrary, at thirty-seven degrees in a temperature of 30.5 degrees.
In this work certain prominent subjects have been selected with which it behoves every one to be acquainted: such, for example, as relate to what may be called our HOUSEHOLD INSTRUMENTS, namely, the Thermometer, the Barometer, and Vernier; the Hydrometer, the Hygrometer; the Tuning-Fork, Musical Glasses and Music generally; the Compass; the Prism, the Telescope, and the Sun-Dial.
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