458 



NATURE 



[September 5, 1901 



heat will cool in the same way as it would if exposed to only 

 one, any more than it is to be expected that a body acted on 

 by two forces will move in the same way as if it were impelled 

 by only one of them. The composition of rates of cooling is like 

 that of velocities in the same straight line ; the resultant rate 

 is the net, or algebraic, sum of all the rates. When the actino- 

 meter is exposed to the sun, its temperature rises at first rapidly, 

 and then more slowly until, if the experiment is sufficiently pro- 

 longed, it becomes stationary. The temperature is noted at 

 equal intervals of time. The sun is screened off, either after the 

 temperature has become stationary or beforehand, and the tem- 

 perature is observed at equal intervals during cooling. Whenever 

 the thermometer is at a higher temperature than its enclosure, 

 it is cooling. Therefore when it is exposed to the sun's rays, 

 and its temperature rises ever so little above that of the enclosure, 

 cooling begins ; and what is observed in the first operation is, 

 not the rate of heating by the sun's rays, but that rate diminished 

 by the rate at which the thermometer is cooling. Hence, when 

 the two series of observations have been made and tabulated, 

 the rate of rise of temperature when that of the thermometer is, 

 say, 2°, 4° or 6' above that of the enclosure is found. Similarly, 

 the rate of fall of temperature when the temperature of the ther- 

 mometer is 2°, 4° or 6° above that of the enclosure during cooling 

 is found. Three pairs of rates are thus obtained. The sums of 

 all three pairs of rates should be alike, and each gives a value of 

 the rate at which the temperature of the actinometer would rise 

 when exposed to the sun if there were no cooling. The rule is 

 the same whether the temperature is allowed to rise to the 

 stationary point or not. A distinction is often made between 

 the static method, when the experiment is continued until the 

 stationary temperature is arrived at, and the kinetic method, 

 when it is interrupted before that temperature is reached. This 

 distinction rests on no substantial difference ; at the same time 

 it is convenient to retain the designations to distinguish the 

 manipulative processes. 



Were the protecting enclosures, such as the double spherical 

 shell packed with melting ice, used by Violle, or the thick 

 metal shell used by Crova, perfectly efficient, then it would not 

 be necessary to make a separate cooling experiment in con- 

 nection with every heating one. The necessity for it is due to 

 the fact that, when the sun's rays are introduced, the temperature 

 of the air in the enclosure no longer is, and it cannot be, at the 

 temperature of the enclosing .shell ; nor can it remain motionless, 

 as it is when at a constant temperature in the shade. These 

 perturbations, which cannot be avoided, so long as there is air 

 in the enclosure, make it impossible to apply a rate of cooling 

 determined beforehand. It is necessary on each occasion to 

 determine the actual integral rate of cooling during the particular 

 experiment. 



If the actinometer could be so arranged that the rate of 

 cooling should not be affected by the introduction or exclusion of 

 the sun's rays, the static method could be adopted without 

 hesitation, and the instrument would become a valuable one for 

 continuous self-recording observations. Their value would be 

 mainly relative. The absolute value of the sun's heat radiation, 

 as it reaches the surface of the earth, has to be determined 

 by other means. When it has been ascertained in the most 

 favourable circumstances it does not vary, excepting in the 

 annual cycle of the earth's revolution. The diurnal variation, 

 as shown by registering actinometers, would have a great local 

 importance. Crova, in the long series of valuable observations 

 which he has made since 1S75 at Montpelier, has, in fact, put 

 this principle in practice. 



Very important observations have been made in the neigh- 

 bourhood of Chamonix by Violle and afterwards by Vallot. The 

 Annates de I'Observatoire mcteorologique du Mont Blanc 

 contain, in vol. ii. , several interesting reports on the results 

 of these observations. They were made simultaneously at 

 Chamonix and at certain stations on Mont Blanc. The first 

 series of observations was made in 1887 on July 28, 29 and 30, 

 and the instruments used were two "absolute actinometers" of 

 Violle (Ann. Chim. Phys. (1879) [5], t. xvii.). 



The great advantage of such experiments is that they are made 

 simultaneously at two stations situated at very different altitudes. 

 At the higher of the two the average barometric pressure is 430 

 millimetres, so that 33/76 of the whole atmosphere are below 

 the observer, and this portion contains nearly all the aqueous 

 vapour. Above him there is a little more than one-half, and 

 that much the simpler and purer half of the atmosphere. In it 

 aqueous vapour is almost absent. The summit of Mont Blanc is 



NO. 1662, VOL. 64] 



4807 metres and the station at Chamonix is 10S7 metres above 

 the sea. The layer of the atmosphere separating them has, 

 therefore, a thickness of 3720 metres, and it can be visited at 

 any point in its thickness. M. Vallot has acquired a personal 

 acquaintance with this layer of air which can only be obtained 

 by devoting a number of years to living in it and observing it. 

 It is this intimate and continuous acquaintance with so large a 

 proportion of the earth's atmosphere that entitles the observa- 

 tions and conclusions of M. Vallot to especially great weight. 



The main results of Vallot's observations are as follows. The 

 ratio between the heat received in the same time by the same area 

 exposed perpendicularly to the sun's rays on Mont Blanc and 

 at Chamonix was found to be o'82 to o'85, which agreed well 

 with the proportion found by Violle in 1S75. The value of the 

 solar radiation found was, however, much lower than that found 

 by Violle. The maximum values observed by Vallot were 

 i"56gr.° C. on Mont Blanc and 133 gr.' C. at Chamonix, 

 whilst Violle found 239 gr.° C. on Mont Blanc and 2-02 gr.° C. 

 at the Glacier des Bossons in the valley. VioUe's observed 

 values are therefore half as great again as Vallot's. No ex- 

 planation of the cause of this discrepancy is offered, but it is 

 pointed out that the values observed by Crova at Montpelier' 

 are more in accordance with Vallot's than with Violle's. They 

 are interesting in themselves and are worth quoting. They 

 relate to the year 1895, the summer of which was very hot. 



Intensity of solar radiation observed by M. Crova at Mont- 

 pelier in 1S95, in gramme-degrees per square centimetre per 

 minute : — • 



The subject was taken up again by Vallot in 1891, and this 

 time he used the mercury actinometer of Crova (Ann. Chim. 

 Phys. 1S77 [5] xi., 461). 



The result of the experiments in 1891 was in the main con- 

 firmatory of those obtained in 18S7. In the following table the 

 intensities of solar radiation on September 19, 1891, are given 

 as observed on Mont Blanc and at Chamonix : — 



The mean valiie of the ratio of the intensities is 0'84, as 

 before. The values of the intensity of radiation are rather 

 lower than those found in 18S7. 



In the year 1896 Prof. Angstrom, of Upsala, made observa- 

 tions on the peak of Tenerife with a special form of actino- 

 meter depending on the heating of metal plates. He made 

 observations at three different elevations, namely, at Guimar, 360 

 metres, Canada, 212$ metres, and at the summit, 3683 metres. 

 Reduced to a uniform thicknessof one atmosphere corresponding 

 to a pressure of 760 mm., the intensity of radiation by the ver- 

 tical sun was found to be at Guimar 1-39, at Caiiada 1-51, 

 and at the summit I '54 gramme-degrees per square centimetre 

 per minute. These values agree more closely with the values 

 found in 1887 by Vallot than with those of 1891. But the 

 values found by Crova, Vallot and Angstrom are all of the same 

 order. 



The writer's observations with the steam calorimeter in Egypt 

 in May 1S82 were undertaken with the object of ascertaining the 

 maximum rate of distillation near the sea. level under the most 

 favourable circumstances. This occurred during the forenoon of 

 May 18, when the meteorological conditions were as favourable 

 as they could be. The sun shone steadily in a cloudless sky. 



