44 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



vature, it will be evident that, in comparing its indi- 

 cation with solar radiation, we must not overlook the 

 fact that the sun, in place of presenting a concave 

 spherical surface, the focus of which is situated at 

 the boundary of the earth's atmosphere, presents a 

 convex semi-spherical face. In consequence of this 

 the sun being 852,534 miles in diameter his limb 

 will be 426,292 miles farther off than the nearest 

 point of his face. The eccentricity of the earth's 

 orbit being 0.01679, while the mean semi-diameter is 

 91,430,000 miles, it follows that, during the summer 

 solstice, the distance from the earth to the centre of 

 the sun will be 92,964,800 miles. Deducting the 

 radius of the sun, AVC ascertain that the distances to 

 the sun's limb and to the centre of his face are re- 

 spectively 92,964,800 and 92,538,500 miles. Invert- 

 ing the square of these distances, it will be found, 

 therefore, that the intensity of radiation from the 

 limb of the sun, compared with the central part, suf- 

 fers a reduction of 0.0086. The temperature at the 

 boundary of our atmosphere being 84.840, it will 

 thus be seen that th.e reduced intensity under consid- 

 eration amounts to 0.0086 x 84.84 = 0.51 Fahr. 

 The observed diminution of the intensity of the 

 radiant heat emanating from the limb of the sun be- 

 ing more than this, it will be seen that we may, with- 

 out material error in our calculations, regard the en- 

 tire surface represented by the area ot the great circle 

 of the sun as being equidistant 92,964,800 miles 

 from the earth. 



It results from previous demonstrations that the 

 temperature of spherical radiators, transmitting 

 equal intensities to their foci, are inversely as the 

 square of the sines of half of the angles which they 

 subtend, that is, the angles formed by the axis of the 

 radiator and the heat-ravs projected from the circum- 

 ference to the focus, \Ve know from previous ex- 

 planations that, owing to the great distance in con- 

 nection with the diminished intensity of the rays 

 emitted by its limb, the radiant power of the sun 

 will be identical with that of a spherical radiator 

 whose focus is situated at the boundary of the earth's 

 atmosphere. Consequently, as the spherical radiator 

 of the solar pyrometer, the tempeiature of which is 

 163.9, transmits to its focus an intensity of 12.2, 

 we are enabled to calculate what temperature the 

 sun must possess in order to transmit an intensity 

 of 12.2 to the boundary of our atmosphere. The 

 angle subtended by the sun is 32' 1" ; that^snbtended 

 by the radiator of the pyrometer 32 15', the ratio 

 of the square of the sines of half of these angles 

 being 1 : 3,567.7. Accordingly, the sun, in order to 

 produce by its radiant heat a temperature of 12.2" at 

 the boundary of the atmosphere of the earth, must 

 possess a temperature of 3,567.7 times greater than 

 that of the spherical radiator of the pyrometer. This 

 latter temperature being 163.9, that of the sun can- 

 not be less than 3,567.7 x 163.9 = 584,746, in order 

 to transmit an intensity corresponding with a ther- 

 mometric interval of 12.2 on the Fahrenheit scale. 

 But solar intensity at the boundary of our atmos- 

 phere, as demonstrated by my actinometer observa- 

 ft4 84 



tions, is 84.84, hence 



6.95 times greater than 



that transmitted by the radiator of the pyrometer to 

 its focus. The temperature of the sun, therefore, 

 cannot be less than 6.95 x 584,746 = 4,063,984 Fahr. 



From other considerations given at length 

 in his papers on the subject, the author deduces 

 that the temperature at the boundary of the 

 solar atmosphere is 2,852,865 Fahr. The 

 mean of the two temperatures here given 

 one at the sun itself, and the other at the 

 boundary of its atmosphere is 3,468,429. 



Subsequently, Captain Ericsson invented 

 another apparatus, to corroborate the experi- 

 ments made with the first. In this the heat 

 was raised to a much higher pitch than 212 



ranging from 1,290 Fahr. to 2,190, but the 

 principle of the operation is the same as that 

 of the before-mentioned. From this he ascer- 

 tained the temperature of the sun itself to be 

 4,451,924 Fahr., a result differing not greatlv 

 from that obtained by the first process. 



Influence of Solar Heat on the EartJCs Ro- 

 tary Velocity. This difficult problem has also 

 received the profound attention of Captain 

 Ericsson, and, like his investigation into the 

 solar temperature, is characterized by great 

 originality and fertility of illustration. He 

 undertakes to show that the sun's radiant heat 

 develops forces capable of diminishing percep- 

 tibly the earth's rotary velocity, and that un- 

 less the retarding influences of solar heat are 

 counteracted by some cosmical force, of which 

 we now have no knowledge, the rotary velocity 

 of this planet will be considerably reduced in 

 the course of time. He speaks of two classes of 

 agencies through which this change is made. 

 One is animate or muscular energy, and the 

 force generated by heat from the combustion 

 of organic matter, both resulting indirectly 

 from the sun's radiant heat. Under this head 

 he refers to man's work on the earth ; for ex- 

 ample, the muscular exertions of the ancient 

 Egyptians, in moving from a distance the mill- 

 ions of tons of matter contained in the pyra- 

 mids, disturbed the previous balance of the 

 rotating mass, causing a tendency to check the 

 earth's rotary velocity and increase the length 

 of the day. The building of great cities from 

 materials taken from below the surface, and 

 raised to a considerable, height above it, has 

 a similar effect. But a retarding agency of 

 greater importance, indirectly resulting from 

 the solar heat, is the constant moving of solid 

 and sedimentary matter in rivers to positions 

 nearer the equator, and hence at a greater 

 distance than before from the axis of rotation. 

 The author presents a series of tables of 

 rivers in the two hemispheres flowing toward 

 the equator, and estimates the retardation in 

 foot-pounds per second both from the trans- 

 ported sediment and the water itself. We can 

 here give only his argument as applied to the 

 Mississippi River. He says: 



The elaborate report of General Humphreys to the 

 Bureau of Topographical Engineers, Washington, 



shows that the average quantity of earthy matter car- 

 >f Mexico, partly suspended in the 



ried into the Gulf of . 



water and partly pushed along the bottom of the 

 river by the current, amounts for each twelve 

 months to 903,100,000,000 pounds. This enormous 

 weight of matter is contributed by numerous largo 

 branches and upward of 1,000 small tributaries. 

 The main distance along the streams, which the sedi- 

 ment is carried in its course to the sea, exceeds 1,500 

 miles. The distance which determines the amount 

 of force tending to check the earth's rotation is ob- 

 viously shorter. 



The maps of the Mississippi Kiver basin, accom- 

 panying General Humphreys' s report, show that its 

 centre is situated 7 10' west of the mouth of the main 

 river, and 11 15' north of the same, in latitude 40 15'. 

 It will be found, on inspecting the accompanying 

 section of the earth, that, agreeable to the stated lati- 

 tudes, the centre of the Mississippi basin rotates in 

 a circle of 15,784,782 feet radius, and that its velocity 



