Sept. ii, 1884] 



NA TURE 



467 



the Newtonian doctrine, the temperature to which it was 



4 2 X 1,680,000 _ 

 subjected indicated a solar intensity of —? 



2,986,000° F. 



The writer has established the correctness of the 

 assumption that " the temperature is as the density of 

 the rays," by showing practically that the diminution 

 of solar temperature (for corresponding zenith distances) 

 when the earth is in aphelion corresponds with the 

 increased diffusion of the rays consequent on increased 

 distance from the sun. This practical demonstration, 

 however, has been questioned on the insufficient ground 

 that " the eccentricity of the earth's orbit is too small and 

 the temperature produced by solar radiation too low " to 

 furnish a safe basis for computations of solar temperature.' 



In order to meet the objection that the diffusion of the 

 rays in aphelion do not differ sufficiently, the solar 

 pyrometer has been so arranged that the density, i.e. 

 the diffusion of the reflected rays, can be changed from 

 a ratio of 1 in 5040 to that of 1 in 10,241. This has 

 been effected by employing heaters respectively 10 inches 

 and 20 inches in diameter. With reference to the " low " 

 solar temperature pointed out, it will be perceived that 



the adopted expedient of increasing the density of the 

 rays without raising the temperature by converging 

 radiation, removes the objection urged. 



Agreeably to the dimensions already specified, the area 

 of the 10-inch heater acted upon by the reflected solar 

 rays is 331 '65 square inches, the area of the 20-inch 

 heater being 673-9 square inches. The section of the 

 annular sunbeam whose direct rays act upon the poly- 

 gonal reflector is 3130 square inches, as before stated. 



Regarding the diffusion of the solar rays during the 

 investigation, the following demonstration will be readily 

 understood. The area of a sphere whose radius is equal 

 to the earth's distance from the sun in aphelion being to 

 the sun's area as 2iS'i- to I, while the reflector of the solar 

 pyrometer intercepts a sunbeam of 3130 square inches 

 section, it follows that the reflector will receive the radiant 



heat developed bv J ,? ,= 

 ' 21S1- 



o'o658 square inch of the solar 



surface. Hence, as the 10-inch heater presents an area of 

 331 '65 square inches, we establish the fact that the re- 

 flected solar rays, acting on the same, are diffused in the 



ratio of 33165 to 0-0658, or , - = 5040 to 1 ; the 



diffusion of the rays acting on the 20 inch heater being as 

 673-9 



673-9 t0 o 0658, or 



0-0658 



10,241 to 1. 



The atmospheric conditions having proved unfavourable 

 during the investigation, maximum solar temperature was 

 not recorded. Accordingly, the heaters of the solar pyrometer 

 did not reach maximum temperature, the highest indica- 

 tion by the thermometer >f the small heater being 336:5, 

 that of the large one being 200°'5 above the surrounding 

 air. No compensation will, however, be introduced on 

 account of deficient solai Leaf, the intention being to base 

 the computation of solar emperature solely on the result 

 of observations conducted at New York during the 

 summer solstice of 1SS4. It will be noticed that the 

 temperature of the large heater is proportionally higher 

 than that of the small heater, a fact showing that the 

 latter, owing to its higher temperature, loses more heat 

 by radiation and convection than the former. Besides, 

 the rate of cooling of heated bodies increases more rapidly 

 than the augmentation of temperature. 



The loss occasioned by the imperfect reflection of the 

 mirrors, as before stated, is 0-235 of the energy transmitted 

 by the direct solar rays acting on the polygonal reflector, 

 hence the temperature which the solar rays are capable of 

 imparting to the large heater will be 2oo°"5 X 1-235 = 

 247°-6l/ ; but the energy of the solar rays acting on the 



reflector is reduced 0^207 by atmospheric absorption, con- 

 sequently the ultimate temperature which the sun's radiant 

 energy is capable of imparting to the heater is 1-207 X 

 247°-6i7 = 29S°S7 F. It is hardly necessary to observe 

 that this temperature (developed by solar radiation dif- 

 fused fully ten-thousandfold) must be regarded as an 

 actual temperature, since a perfectly transparent atmo- 

 sphere, and a reflector capable of transmitting the whole 

 energy of the sun's rays to the heater, would produce the 

 same. 



The result of the experimental investigation carried out 

 during the summer solstice of 1884 may be thus briefly 

 stated. The diffusion of the solar rays acting on the 20- 

 inch heater being in the ratio of I to 10,241, the tempera- 

 ture of the solar surface cannot be less than 29S°'87 

 X 10,241 = 3,060,727° F. This underrated computation 

 must be accepted unless it can be shown that the tem- 

 perature produced by radiant heat is not inversely as the 

 diffusion of the rays. Physicists who question the ex- 

 istence of such high solar temperature should bear in mind 

 that in consequence of the great attraction of the solar mass, 

 hydrogen on the sun's surface raised to a temperature of 

 4000' C, will be nearly twice as heavy as hydrogen on the 

 surface of the earth at ordinary atmospheric tempera- 

 tures : and that, owing to the immense depth of the solar 

 atmosphere, its density would be so enormous at the stated 



