Nephoscope.—The direction and apparent velocity of the
motion of a cloud are best observed by means of the nephoscope,
which has now become a necessary item in the outfit of any
first-class meteorological station. Among the various forms
of this instrument are the nephodoscope of Fornioni, the marine
nephoscope of Fineman, the simple mirror with attachments
used by Clayton, the cloud camera of Vettin, and the alt-azimuths
of Mohn and Lettry. The most perfect form for
use on land is that devised by Professor Marvin in 1896 for
the U.S. Weather Bureau stations (see fig. 2); while the
most convenient for use at sea is that devised and used
in 1889 by Professor Abbe on the cruise of the U.S. ship
“Pensacola” to the west coast of Africa, but first described
in the report of the International Meteorological Congress
held at Chicago in August 1893.
Fig. 2.—Marvin’s Nephoscope.
The construction of this instrument is shown in figs. 3, 4, 5. In using it the observer looks down upon a horizontal mirror and observes the reflection of the cloud. By moving his eye he brings any cloudy point into coincidence with, the reflection of a small fixed spherical knob K above the mirror, and keeps the images of the knob and the cloud coincident as they pass from the centre of the mirror to its edge. This line of motion shows the azimuth of the horizontal component of the cloud’s motion. The course of the vessel is shown by the compass card and lubber line AF seen below the mirror. The apparent angular velocity of the cloud, as it would be if the cloud started from the zenith, is obtained by counting the seconds that elapse between its passage from the centre to the edge, or to a small circle inscribed within the edge. With Marvin’s nephoscope two observers a short distance apart may easily determine the apparent altitude, and azimuth, and motion of any cloud, whence its true altitude and velocity may be computed. But when the observer uses Abbe’s marine nephoscope on a vessel which is itself in motion he observes the resultant of his own motion and that of the cloud. If his vessel is under his control, so that he may change its velocity or direction at will, he easily determines this resultant for two different courses, and obtains data by which he is enabled to calculate the real altitude and velocity of the cloud in terms of his own velocity. As the marine nephoscope can be used on a wagon moving rapidly over a smooth road, or in a small boat on a smooth pond, almost as well as on a larger sea-going vessel, it becomes an instrument of universal application for cloud study. It is also equally convenient for observing the positions of auroras, halos, meteors, and other special phenomena. For the international work undertaken during the year 1898 the photographic camera established upon an alt-azimuth mounting, or the so-called photogram-meter, was especially developed. In this apparatus photographs of the clouds are taken simultaneously at two or more stations, and in each case the centre of the photographic plate has its altitude and azimuth determined. From this centre one can measure on the plate the additional angles required in order to fix the altitude and azimuth of any point that is photographed, and thus the dimensions of the whole visible cloud and its internal or differential motions can be determined, as well as its general motion. During the years 1896–1898 about twenty stations were occupied throughout the world for the purpose of determining accurately the altitudes and motions of every layer of cloud.
Sunshine Recorder.—The ordinary meteorological record specifies the proportion of sky that appears to be covered with cloud, or the so-called cloudiness, usually expressed in tenths. The observer generally confines his attention to that portion of the sky within sixty degrees of the zenith, and ignores the lower zone, since the clouds that are found therein are often at so great a distance from him that their record is not supposed to belong to his locality. As the cloudiness—or its reciprocal, the sunshine—is supposed to be the most important item in agricultural climatology, and is certainly very important for dynamic meteorology, it is usually considered desirable to obtain more complete records than are given by only one or two specified hours of observation. To this end apparatus for recording sunshine, or, rather, the effect of cloudiness, is widely adopted. At least three forms are worth describing as being extensively used.
The Jordan photographic sunshine recorder consists of a cylinder enclosing a sheet of sensitive paper; the sun’s rays penetrate through a small aperture, and describe a path from sunrise to sunset, which appears on this sheet after it has been properly washed with the fixing solution. Any interruption in this path, due to cloudiness or haze, is of course clearly shown, and gives at once the means of estimating what percentage of the day was clear and what cloudy. The modified form of the instrument devised by Professor Marvin has been used for many years at about forty Weather Bureau stations, but the original construction is still employed by other observers throughout the world. The Stokes-Campbell recorder consists of a globe of glass acting as a burning-glass. A sheet of pasteboard or a block of wood at the rear receives the record, and the extent of the charring gives a crude measure of the percentage of full or strong sunshine. Many of these instruments are used at stations in Great Britain and the British colonies. The Marvin thermometric sunshine recorder consists of a thermometer tube, having a black bulb at the lower end and a bright bulb at the other. The excess of temperature in the black bulb causes a thread of mercury to move upwards, and for a certain standard difference of temperature of about 5° F., such as would be produced by the sun shining through a very thin cloud or haze, a record is made by an electric current on a revolving drum, and simply shows when during the day sunshine of a certain intensity prevailed, or was prevented by cloudiness, D. T. Maring, in the U.S. Monthly Weather Review for 1897, described an ingenious combination of the thermometer and the photographic register of cloudiness which is worthy of further development. It gives both the quantity of cloudiness and intensity of the sunshine on some arbitrary relative scale.
The intensity of the sunshine, as sometimes employed in general agricultural studies, is crudely shown by Violle’s conjugate bulbs, which are thin copper balls about 3 in. in diameter, one of them being blackened on the outside and the other gilded. When exposed to the sunshine the difference in temperature of the two bulbs increases with the intensity of the sunshine, but as the difference is dependent to a considerable extent on the wind, the Violle bulbs have not found wide application. The Arago-Davy actinometer, or bright and black bulbs in vacuo, constitutes a decided improvement upon the Violle bulbs, in that the vacuous space surrounding the thermometers diminishes the effect of the wind. The physical theory involved in the use of the Arago-Davy actinometer was fully developed by Ferrel, and he was able to determine the coefficient of absorption of the earth’s atmosphere and other data, thereby showing that this apparatus has considerable pretensions to accuracy. In using it as contemplated by Arago and Davy and by Professor Ferrel, we read simply the stationary temperature attained by the bright and black thermometers at any moment, whereas the best method in actinometry consists in alternately shading and exposing any appropriate apparatus so as to determine the total effect of the solar radiation in one minute, or some shorter unit of time; this method of using the Arago-Davy actinometer was earnestly recommended by Abbe in 1883, and in fact tried at that time; but the apparatus and records were unfortunately burned up. This so-called dynamic, as distinguished from the static, method was first applied by Pouillet in 1838 in using his pyrheliometer, which was the first apparatus and method that gave approximate measures of the radiant heat received from the sun. In order to improve upon Pouillet’s work more delicate apparatus has been constructed, but the fundamental methods remain the same. Thus Ångström has applied both Langley’s bolometer and his own still more sensitive thermoelectric couple and balance method; Violle uses his absolute actinometer, consisting of a most delicate thermometer within a polished metal sphere, whose temperature is kept uniform by the flow of water; while Crova, with a thermometer within an enclosure; of uniform temperature, claims to have attained an accuracy of one part in a thousand. Chwolson has reviewed the whole subject, of actinometry, and has shown the greater delicacy of his own apparatus, consisting of two thin plates alternately exposed to and shielded from sunshine, whose differences of temperature are measured by electric methods.
As none of the absolute methods for determining the solar radiation in units of heat lend themselves to continuous registration, it is important to call attention to the possibility of accomplishing this by chemical methods. The best of these appears to be that devised by Marchand, by the use of a device which he calls the Phot-antitupimeter. In this the action, of the sunlight upon a
solution of ferric-oxalate and chloride of iron liberates carbonic acid