550 



NA TURE 



[April 4, 1901 



of terrestrial gravity at the earth's surface, one cubic metre of 

 cold solid iron on the sun's surface would exercise a pressure of 

 210,000 kilogrammes. To lift this mass through one kilometre 

 against solar gravity would involve the expenditure of 210 x 10® 

 kg.m. of work : and if this amount of work were done in one 

 minute, the engine employed would have to develop 46,667 

 horse-power. 



Further, the heat which is equivalent to 210 x 10^ kg.m. 

 of work is 494, x 00 kilogramme-degrees (Kg. ° C). When iron 

 is burned in oxygen so as to form the magnetic oxide, the heat 

 evolved is given by the thermochemical equation 

 Fes + 04 = FejOi + 2647 Kg." C. 



Using this constant, we find that the mass of iron which by its 

 combustion would furnish the above amount of heat, would 

 weigh on the surface of the earth 313*5 kilogrammes, and would 

 occupy a volume of 0*04 18 cubic metre, or i square metre x 

 4'i8 centimetres. Therefore the heat required could be pro- 

 duced by burning 4'i8 centimetres of liquid iron on a hearth of 

 I square metre per minute. With a supply of oxygen of high 

 tension this would not seem to be an insurmountable task. This 

 is put forward only as an illustration, and in no way as an 

 explanation of the source of the heat of the sun. 



With this caution, however, I should like to call attention to 

 a coincidence. 



The specific heat of Fe304 is o"i678, and its molecular weight 

 232, whence the water value of the gr. molecule is 38*93 grs. 

 The molecular heat of combination is 264,700 grs.° C. Dividing 

 this number by 38*93 we get 6800° C. as the temperature of the 

 FcgO^ produced. Adding 273, we have 7073° C. as the abso- 

 lute temperature which may be produced. In a recent work ^ 

 Scheiner gives 7010° C. as the most probable effective absolute 

 temperature of the sun. 



Whilst the maximum value recorded by the calorimeter is the 

 most important for the determination of the sun's heating power, 

 the other values obtained are of use for testing the working of 

 the instrument. Ihe principal disturbing element is wind. 

 During the forenoon of the i8th there was an almost complete 

 absence of wind. We take the observations of that forenoon, 

 neglecting those that show a diminution of intensity as noon is 

 approached, because the sun's heating power cannot diminish 

 as noon is approached. They are collected in the following 

 table. In the first column a is the mean time corresponding to 

 the mean rate of distillation under d. Under b we have the 

 sun's zenith distance at this time, and under c the secant of this 

 angle, so that <r = sec b. Under rfare the mean rates of distilla- 

 tion, in c.c. per minute, for quantities of 20 c.c. collected. 

 Under e are these rates reduced to their value per square metre 

 per minute, e=\\ '06 d. Under f we have the values of e cor- 

 rected for the obliquity of the sun's rays. For this purpose the 

 formula given by Herschel in his " Meteorology" is used.^ It 

 is, using the letters in our table. 



e =f{^Y whence/: 



'lir 



In this equation | is the transmission coefficient of the atmo- 

 sphere ; therefore f is really the solar constant expressed in 

 cubic centimetres water evaporated per square metre per minute. 

 Under g it is given in grs.° C. per square centimetre per minute, 

 whence <§■= 0*0535/ -^s we have assumed f to be the trans- 

 mission coefficient of the atmosphere, and have found the 

 vertical intensity of the sun's rays outside of the atmosphere, 

 we obtain at once its intensity at the sea-level h — \f. This is 

 expressed in cubic centimetres water evaporated per square 

 metre per minute, and it is practically unaffected by the value 

 which we accept as the transmission coefficient of the 

 atmosphere. 



The figures in this table show that the values of the heating 

 effect of the rays of the vertical sun, deduced from observations 

 made when the sun was at zenith distances ranging between 

 10° and 44°, are practically identical. This affords very strong 

 evidence that the calorimeter is a trustworthy instrument. 



Reverting to our maximum value with an allowance for in- 

 strumental imperfection, if we take one horse-power per square 

 metre as the intensity of the rays of the vertical sun at the sea- 

 level, their intensity | outside of the atmosphere is i *5 horse- 

 power per square metre, using Herschel's value for the trans- 

 mission coefficient. This is equivalent to 15,882 grs.° C. per 

 square metre, or 1*588 grs." C. per square centimetre per 

 minute. In round numbers we obtain i "6 for the value of the 

 solar constant. While it is possible that this value may be a 

 little too low, reasons are given in the paper for believing that 

 the values commonly received, which lie between 3 and 

 5 grs.° C. per square centimetre per minute, are much ex- 

 aggerated. 



Observations made during the Eclipse on the morning of \ 

 May 17, 1882. j 



The calorimeter was directed to the sun as soon after totality 

 as possible. At 8h. 34m. the sun was totally eclipsed ; at 8.51 

 the calorimeter was directed to the sun, but no boiling took 

 place. At 8.58 the vvater began to " sing" ; at 9.1 it boiled ; 

 at 9.3 it was boiling briskly, but it was not till 9.17 that the 

 first drop of distillate fell into the receiver. By 9. 19*5 i c.c. 

 had passed, and between 9.21 and 9.29*5 5 c.c. passed. 



The observations made at this time are collected in the fol- 

 lowing table. In the first column is the apparent solar time of 

 each observation, in the second column is the volume of distil- 

 late collected at that time, in the third column is the mean date 

 of collecting each portion, in the fourth column is the date 

 stated in minutes after totality, in the fifth column is the average 

 rate of distillation in c.c.'s per minute during the interval, and 

 in the sixth column is the percentage of the sun's disc exposed. 



^"Strahlung und Temperatur der Sonne." Von Dr. J. Scheiner, 

 Leipzig, iSgp, p. 39. 



- " ^leteorology," by Sir John Herschel, Bart., Edinburgh, 1861, p. to. 



From this table we see that when distillation has begun, it 

 increases at a much greater rate than the exposed sun's surface. 

 This must be so in the early stages, because we see that it is not 

 till 26 minutes after totality, and when already 33 per cent, of 

 the sun's surface has been uncovered, that the water in the 

 boiler boils, and it takes 16 minutes more before any distillate is 

 collected. Even when 50 per cent, of the sun is exposed, the 

 rate of distillation is only 0*4 c.c. per minute. After this more 

 weight may be attached to the observations, but their numerical 

 significance is not great. Still, they show that useful informa- 

 tion could be obtained by arranging lor making trustworthy 

 observations during the progress of an eclipse. 



In the case of a total eclipse there must be an interval during 

 which the sun cannot keep steam, however large the reflector 



NO. 1640, VOL. 63] 



