September 5, 1901] 



NA TURE 



457 



is j^reatest when it strikes it perpendicularly. At every moment 

 the sun is vertical over one spot or another of the earth's surface. 

 Therefore our first step should be to choose a locality where the 

 sun passes through the zenith at mid-day. 



Before reaching the sealevel the sun's rays have to pass 

 through the whole thickness of the atmosphere. It is a matter 

 of every-day observation that the atmosphere varies in trans- 

 parency. The second condition is therefore to put ourselves in 

 the position of greatest advantage as regards atmospheric con- 

 ditions. Clouds and similar visible obstructions are of course 

 excluded. The air should be motionless, the sky should be 

 clear and of a deep blue colour in the regions remote from the 

 sun and should contain nothing that can be called haze, or 

 that interferes with the definition of the sun or other heavenly 

 bodies. 



From inspection alone we can only approximately ascertain' 

 what are the most favourable meteorological conditions. For 

 this reason it is necessary to multiply observations and never to 

 miss line weather. In the end we cannot fail to approach 

 nearer and nearer to the exact determination of the maximum 

 heating power of the sun on the earth's surface at or near the 

 sea-level, in so far as the degree of perfection of our instrumental 

 resources permits. This limitation imposes on us the duty to 

 continue observations, not only until the best natural conditions 

 have been found, but also-so long as the instruments or experi- 

 mental methods appear to be capable of improvement. If we 

 suppose for one moment that we have arrived at the point where 

 no further improvement is possible, then the result of our work 

 is the determination of the rale at which unit area of the earth's 

 surface at or near the sea-level receives heat from the vertical 

 sun in unit time. 



There is no question here of how much is lost on the way 

 from the sun. All that is sought, and the most that is ascer- 

 tained, is how much arrives. If we multiply this by the area 

 included in the great circle of the earth we have the amount of 

 r.adiant heat which we can count on as being supplied to the 

 whole earth in unit of time. This is the constant which is of 

 greatest importance in physical geography. 



When we have ascertained the supply of radiant heat which 

 reaches the earth's surface, we have to inquire what becomes of 

 it. If the heat were to accumulate the world would become 

 uninhabitable. It cannot be doubted that long ago the earth, 

 in this respect, arrived at a condition of equilibrium which is 

 maintained with very slight oscillations. The fundamental 

 principle of this state of equilibrium is that the heat which the 

 whole earth receives from the sun in the course of a year also 

 leaves it in the course of a year, .so that, taking one year with 

 another, the sum of the heat remains the same. 



When we study the details of the annual dissipation of heat 

 we find that the atmosphere, and especially the aqueous vapour 

 in it, performs a very important part. Although practically 

 transparent to the heat-rays passing from the sun to the earth, it 

 is very opaque to those leaving the earth to pass outwards. 

 They are powerfully absorbed and the temperature of the atmo- 

 sphere is thus raised considerably above that which it would 

 have if it were as transparent to the leaving rays as it is to the 

 entering ones. This has no eflect in permanently detaining .any 

 of the year's supply, it still disappears in the year, but not 

 before it has produced important climatic effects. 



We see in this difierential behaviour of the atmosphere to- 

 wards the incoming and the outgoing rays an example of 

 Kirchoff's law, in virtue of which a body absorbs by preference 

 the rays which it itself emits. It is exceedingly unlikely that 

 any portion of the ra>s coming directly from the sun proceed 

 from highly heated water or water vapour ; we should therefore 

 not expect the water vapour in the atmosphere to absorb them 

 to any appreciable extent. When, however, they strike the 

 surface of the earth, whether it be land or sea, they are 

 abund.antly absorbed. The blue water of the ocean transmits 

 the sun's visible rays to a considerable depth. In experiments 

 made by the writer on board the Challenger, a white surface, 

 about four inches square, was clearly visible at a depth of 25 

 fathoms. The total length of the path of the incident and re- 

 flected ray was 50 fathoms ; therefore the sun's rays which 

 strike the sea have a thickness of at least 100 metres to work 

 on. When they strike the land, the direct effect is superficial, 

 but the absorptive power of a surface of soil is very much 

 greater than that of a surface of water, and it frequently attains 

 a very high temperature. Even in the driest countries the soil 

 is moist, and it may be that, ultimately, the surface of every 



NO. 1662, VOL. 64] 



particle of the soil is a water surface. Whether this be so or 

 not, when a land surface cools, the heat of low refrangibility 

 which it radiates proceeds to a very large extent from water, and 

 it is accordingly abundantly absorbed by the water vapour in the 

 lower layers of the atmosphere. In the absence of mechanical 

 mixture by wind, these layers can lose it only by passing it on 

 by radiation to higher Layers which contain moisture, whence 

 it ultimately escapes into space. This accumulating function 

 of the atmosphere provides that while every portion of the earth's 

 surface receives heat intermittently it loses it continuously. 



As the heat of the atmosphere is due to contact with, or 

 radiation from, the surface, it must be taken from the supply 

 that reaches the surface of the earth. Further, wind and all 

 mechanical atmospheric effects are due to differences of density, 

 and these are produced, not only by the thermal expansion and 

 accompanying rise of temperature of the air, but also, and with- 

 out change of temperature, by the mixture with it of a lighter 

 gas. Such a gas is the vapour of water, and the water which 

 supplies it is at the level of the sea. Therefore the sun's heat 

 which arrives at the surface of the earth at or near the sea- 

 level has to maintain not only the temperature of the surface 

 of the globe, it has also to maintain all the mechanical mani- 

 festations of the air and the ocean. This is the ground for 

 asserting, as above, that the only constant which is of interest 

 in terrestrial physics is the rate at which the vertical sun heats 

 unit area of the earth's surface at the sea-level. 



The instruments used for measuring the thermal effect of the 

 sun's rays must fulfil certain conditions. The area of the sheaf 

 or bundle of rays collected must be accurately known ; and 

 provision must be made for the exact measurement of the thermal 

 effect produced by them in a given time. The thermal effect 

 produced is measured by a mass of some substance and either 

 by the change of temperature produced in it or by the change 

 of its state of aggregation. Actinometers, such as those of 

 Herschel, Pouillet, VioUe, Crova, are instruments of the first 

 kind. The ice calorimeter used by Exner and Rcintgen and the 

 steam calorimeter of the writer are instruments of the second 

 kind. The thermal mass of the substance affected is conveniently 

 expressed in terms of the thermally equivalent weight of water, 

 which is called its water value. In the .actinometer the change 

 of temperature is either measured by a separate thermonieter or 

 the actinometer is itself a thermometer the calorimetric con- 

 stants of which have been ascertained. In instruments of the 

 second class no thermometer is required: the thermal effect is 

 measured by the mass of water-substance which changes its state 

 in a given time either from ice tc water or from water to steam, 

 both being at the same temperature. In the ice calorimeter 

 the quantity of liquefaction is measured by the change of volume, 

 as in Bunsen's calorimeter ; in the steam calorimeter the gen- 

 eration of steam is measured by the weight or volume of the 

 distilled water produced. The steam calorimeter was described 

 recently in Natl-re (vol. Ixiii. p. 548), and it is unnecessary to 

 repeat it here. It acted quite satisfactorily in the writer's hands 

 in Egypt in May 18S2, and it has since been giving good results 

 in the hands of Mr. Michie Smith at the observatory of Kodai- 

 kanal in South India, at an elevation of about 7000 feet above 

 the sea. Theoretically, the ice calorimeter is as good as the 

 steam calorimeter, but in applying it to the measurement of the 

 sun's radiant heat it has a practical defect. At the moment 

 before exposure, the ice in the calorimeter is frozen to the inner 

 surface of the metal plate, the outer surface of which receives the 

 sun's rays. The first effect of expo.sure to the sun is that the ice 

 is detached from the plate. The intervening water introduces 

 perturbations which are not easily allowed for. 



The fundamental principle of the actinometer is analogous to 

 Newton's second law of motion ; when a body is engaged in the 

 exchange of heat between itself and any number of other bodies, 

 each exchange takes place independentlv of the others. The 

 rate of exchange in each case depends on the difference of tem- 

 perature between the two bodies and takes place on the principle 

 that equal fractions of heat are lost or gained in equal times. A 

 body cooling in the air is always subject to at least two quite 

 independent sources of loss of heat, namely, radiation between 

 itself and the surrounding objects and conduction between itself 

 and the contiguous air. In ordinary circumstances the rate 

 of loss of heat by radiation is subject to but little variation, but 

 that due to conduction is subject to continual variation owing to 

 the varying rate at which the air actually in contact with the 

 thermometer is renewed. It is not to be expected that 

 a body subject to at least two independent sources of loss of 



