than the free air. The results of analysis at the end of the experiments showed that under the transparent glass the weight of the roots was the same as in the free air, but the weight of the leaf was much more, the weight of the sugar much less, and the weight of the nitrous salts much greater. Under the black glass the weight of the roots was 4 per cent of that in the free air, and the weight of the leaves was about 25 per cent, the weight of sugar 2 per cent, and the weight of the salts 8 per cent, thus demonstrating an almost complete stoppage of the vital processes.

Evidently the action of these artificial coverings on the experimental plants is perfectly analogous to the action of cloud and fog in

nature.

It is commonly said that on the seacoast the action of the salt brine blown by the wind up over the land is to stunt or prevent vegetable growth, but the same effect must be produced by the absence of sunlight in those regions where fog and cloud prevail.

INFLUENCE OF SHADE ON DEVELOPMENT.

According to Marchand (1875, p. 130), the influence of a diminution of sunlight on the development of the plant is apparent in the relative growth of plants on sunny and cloudy days or in sunny and shady places, but the matter was brought to exact measurement by Hellriegel. His experiments on barley gave him these results:

We see here that plants living in the greenhouse, receiving sunlight that has traversed the glass, have experienced a considerable diminution in their development as compared with those in the free air which experienced the full chemical force of the sunshine. The plants living under glass and in the diffuse light developed only a small quantity of stalk and did not perfect the seed at all.

INFLUENCE OF LONG AND SHORT WAVES OF LIGHT.

Vöchting (1887) investigated the formation of tubers as influenced especially by sunlight. Sachs had maintained that the germination was entirely prevented, or at least went on very slowly, if sunlight,

i. e., short waves, had access to the tubers. Vochting finds that, although the light does delay the growth and diminishes the distance between the tubers, still the supply of water is the important factor. (Wollny, X, p. 230.)

Sachs (1887), as the result of experiments on the effect of ultraviolet radiation upon the formation of buds, states that these rays exert on the green leaves (in addition to the assimilation produced by the yellow and neighboring rays) still another effect that consists in the development of particles that contribute to the formation of blossoms. These bud-forming particles move from the leaves into those parts of the plant where they are to bring about their own development into buds. We therefore now know of three different portions of the solar spectrum having very different physiological influences: The yellow and neighboring rays, which bring about the transformation of carbonic acid or the formation of starch; the blue and visible violet, that act as stimulants to motion; the ultraviolet rays, that produce in the green leaves the material for the formation of buds. (Wollny, X, p. 230.)

INFLUENCE OF DRYNESS AND SUNLIGHT ON DEVELOPMENT OF TUBERS.

In the climate of Germany the flowering of different varieties of potatoes is very much restricted. Only a small number of varieties flower regularly and bear fruit, whereas in Chile the plant flowers abundantly, but the tubers are small; in other words, in the Temperate Zone the formation of tubers is favored at the expense of fertilization; the energy of the one process increases while the other diminishes.

Knight and Langenthal have found that by detaching the young tubers they increase the blooming, and on the other hand, by cutting off the flowers they increase the development of the tubers, thereby largely increasing the harvest. Wollny, in 1886, experimented on four plats, each for many varieties of potatoes. He found that cutting off the flowers increased the crop of tubers as to number, size, and weight, but that something depended upon the time of cropping the flowers, which is best done a considerable time before they arrive at maturity. It seems probable that dryness and sunlight stimulate the formation of flowers, but humidity and cloudiness, at least up to a certain limit, stimulate the formation of tubers. This harmonizes with some recent results obtained by Sachs, who has shown that the ultraviolet rays stimulate the flowering. (Agr. Sci., Vol. II, p. 273.)

Chapter V.

THE METHODS OF MEASURING DIRECT OR DIFFUSE SUNSHINE AS TO INTENSITY OR DURATION.

Sunshine may be measured as to its quality or wave length, its intensity, or its duration. The methods used in measuring either of these must be understood in order to intelligently compare the published observations with phænological phenomena. The following section considers some of the methods of measuring or registering the duration or intensity of sunshine, or the intensity of the skylight, at least in so far as these have been used in agricultural studies. THEORETICAL RELATION OF DIRECT AND DIFFUSED SUNSHINE.

The relative intensity of any radiation may be measured by its heat or light or chemical effect. The insolation received by a horizontal surface, whether directly from the sun or diffusely from the sky, is subject in a general way to calculation, but the irregularities introduced by haze and clouds can not be so calculated and must be observed daily. The following table gives, for a clear blue sky, the values obtained by Clausius for the radiation (S) that falls upon a horizontal surface directly from the sun, and in the third column the diffuse radiation (C) that falls from the whole sky upon that same surface; the total radiation (S+C) is the sum of these two. If, however, the surface is normal to the sunlight, instead of horizontal, it receives the quantity in the fifth column (I) directly from the sun, and (c) which is less than the quantity (C) from the sky, depending upon the altitude of the sun, the total being, as before, the sum of these (I+c). The study of these columns shows us the maximum and minimum amounts of sunshine that may fall upon a given leaf surface, since a leaf will in general be in some position to receive the full sunshine normally to its surface, while others will be horizontal, or vertical, or in the shade, and receive only a part of the diffuse light from the sky.

It is assumed by Radau, in his actinometry (1877), as also by Marié-Davy, that the bright and black bulb thermometers in vacuo, or the so-called "conjugate thermometers," give us the total radiation (C+I) as for the horizontal surface, and that this is the quantity in which vegetation is interested.

TOTAL INSOLATION, DIRECT AND DIFFUSED.

The value of the intensity of the direct solar rays incident normally to any unit surface, as determined by the absolute actinometers of Pouillet, Violle, and others, is not so applicable to the study of the growth of plants as is the sum of the radiation from the sky and other surroundings of the plant, added to the direct solar radiation. Comparative measures made in 1866 by Roscoe, at Manchester; Baker, at Kew; Wollkoff, on the summit of Koenigstuhl, near Heidelberg (altitude, 550 meters), and Thorpe, at Para, have given the following values of relative intensity of radiation at certain moments when the sun's altitude above the horizon was sensibly the same at all the stations. (See Marie Davy, 1882.)

Relative intensity of radiation for equal altitudes of the sun.

At Manchester and at Paris the light that comes from the sky is more than double that which comes directly from the sun. When the sun is hidden by clouds, or even partially veiled, it is the radiation from the sky that is of the most importance to agriculture, and in any case this radiation is far from being negligible.

The Arago-Davy actinometer (believed to have been invented by Arago before 1844, but improved by Marié-Davy and used at the

observatory of Montsouris ever since 1873) is an apparatus that is intended to determine the total solar plus sky radiation that is needed in agricultural physics. A theory of the action of this instrument was devised by Marié-Davy, but the proper method of calculating its results was first developed with exactness by Ferrel, in Professional Papers of the Signal Service, No. XIII (1884), and subsequently in his Recent Advances in Meteorology (Annual Report, Chief Signal Officer, p. 373). His formula, will be given on page 88.

The Arago-Davy actinometer is composed of two mercurial thermometers with very fine tubes, and having spherical reservoirs of equal dimensions, one colorless and the other covered with lampblack. In the empty space above the mercury in the thermometer tubes there is a small quantity of hydrogen or other inert gas. The small quantity of gas left in the tubes of these thermometers has no other object than to prevent the mercury from falling in the tube by the force of gravity when the bulb is turned upward toward the sky. Each thermometer is inclosed in a larger glass tube or cylinder, terminated by a spherical enlargement, in the center of which is placed the center of the bulb of the thermometer. This tube and enlargement constitute the inclosure, and it is exhausted of air as perfectly as possible. The immovability of the thermometer, relative to the walls of its inclosure, is assured by a soldering at the upper extremity of the tube and, at the opposite end toward the reservoir, by two rings of cork held by friction between the interior tube and exterior cylinder. These thermometers, with their respective glass inclosures, are turned up with their bulbs toward the sky, and by means of double clamps fixed parallel to two metallic rods, arranged in the form of a V and turned, the one toward the east, the other toward the west. These metallic rods make an angle with each other of 60°—that is to say, of 30° with the vertical-and are fastened to a support of wood or iron 1.20 or 1.30 meters in height above the earth. The support is solidly planted in the ground in an open place, remote from buildings, plants, or any other obstacle capable of intercepting the direct radiation of the sun. The two thermometers, the envelopes of which are exposed near each other, have necessarily the same temperature and mark the same degree as long as they remain in perfect darkness; but hardly does day begin to break than the thermometer with the black bulb marks a higher temperature than that with a plain glass bulb. The difference in temperature of these two thermometers gives the "actinometric degree" for the moment of observation; that is to say, it serves to measure the intensity with which the radiation strikes the two thermometers and is absorbed by the black bulb; consequently, at least approximately, it serves to measure the intensity with which the