A Treatise on Meteorological Instruments / Explanatory of Their Scientific Principles, Method of Construction, and Practical Utility

by Enrico Angelo Lodovico Negretti; Joseph Zambra

By means of the process of terrestrial radiation ice is artificially formed in Bengal, “where the substance is never formed naturally. Shallow pits are dug, which are partially filled with straw, and on the straw flat pans containing water which had been boiled is exposed to the clear firmament. The water is a very powerful radiant, and sends off its heat into space. The heat thus lost cannot be supplied from the earth—this source being cut off by the non-conducting straw. Before sunrise a cake of ice is formed in each vessel.... To produce the ice in abundance, the atmosphere must not only be clear, but it must be comparatively free from aqueous vapour.”

Considering, therefore, the important consequences attending both terrestrial and solar radiation, it appears to us that observations from radiation thermometers are of much more utility in judging of climate than is usually supposed. These observations are very scanty; and what few are upon record are not very reliable, principally from bad exposure of the instruments, while the want of uniformity in construction may be another cause. Herschell’s actinometer and Pouillet’s pyrheliometer, instruments for ascertaining the absolute heating effect of the sun’s rays, should, however, be more generally employed by meteorologists. In comparing observations on radiation it should be kept in mind, that “the difference between a thermometer which, properly confined [or shaded], gives the true temperature of the night air, and one which is permitted to radiate freely towards space, must be greater at high elevations than at low ones;”[6] because the higher the place, the less the thickness of the vapour-screen to intercept the radiation.

83. Solar Radiation Thermometer.—“As the interchange of heat between two bodies by radiation depends upon the relative temperature which they respectively possess, the earth, by the rays transmitted from the sun during the day, must be continually gaining an accession of heat, which would be far from being counterbalanced by the opposite effect of its own radiation into space. Hence, from sunrise till two or three hours after mid-day, the earth goes on gradually increasing in temperature, the augmentation being greatest where the surface consists of materials calculated, from their colour and texture, to absorb heat, and where it is deficient in moisture, which, by its evaporation, would have a tendency to diminish it.”[7] It is, therefore, important to have instruments for measuring the efficacy of solar radiation, apart from those for exhibiting the temperature of the place in the shade.

Fig. 63.

Fig. 63 shows the arrangement of Negretti & Zambra’s maximum thermometer, for registering the greatest heat of the sun’s direct rays, hence called a solar radiation thermometer. It has a blackened bulb, the scale divided on its own stem, and the divisions protected by a glass shield. In use it should be placed nearly horizontally, resting on Y supports of wood or metal, with its bulb in the full rays of the sun, resting on grass, and, if possible, so that lateral winds should not strike the bulb; and at a sufficient distance from any wall, so that it does not receive any reflected heat from the sun. Some observers place the thermometer as much as two feet from the ground. It would be very desirable if one uniform plan could be recognized: that of placing the instrument as indicated in the figure appears to be most generally adopted, and the least objectionable.

84. Vacuum Solar Radiation Thermometer.—In order that the heat absorbed by the blackened bulb of the solar radiation thermometer may not in part be carried off by the currents of air which would come into contact with it, the instrument has been improved by Messrs. Negretti and Zambra into the vacuum solar radiation thermometer, as illustrated by fig. 64.

Fig. 64.

This consists of a blackened-bulb radiation thermometer, enclosed in a glass tube and globe, from which all air is exhausted. Thus protected from the loss of heat which would ensue if the bulb were exposed, its indications are from 20° to 30° higher than when placed side by side with a similar instrument with the bulb exposed to the passing air. At times when the air has been in rapid motion, the difference between the reading of a thermometer giving the true temperature of the air in the shade, and an ordinary solar radiation thermometer, has been 20° only, whilst the difference between the air temperature and the reading of a radiation thermometer in vacuo has been as large as 50°. It is also found that the readings are almost identical at distances from the earth varying from six inches to eighteen inches. By the use of this improvement, it is hoped that the amounts of solar radiation at different places may be rendered comparable; hitherto they have not been so; the results found at different places cannot be compared, as the bulbs of the thermometers are under very different circumstances as to exposure and currents of air. Important results are anticipated from this arrangement. The observations at different places are expected to present more agreement. Observers would do well to note carefully the effect of any remarkable degree of intensity in the solar heat upon particular plants, crops, fruit or other trees.