NATURE 



\_Nov. 1 6, I Syr 



his recent work " Le Soleil " (p. 267). " During a great 

 number of observations made at Rome," says the author, 

 "the difference between the two temperatures (that indi- 

 cated by the thermometer exposed to the sun and that of 

 the surrounding casing), was l2-o6° (2170° Fah.) ; during 

 days when the sl<y was clearer, it rose to 14°." Conse- 

 quently, the highest temperature indicated by the instru- 

 ment referred to, was 25^2^ Fah., against 66'04°, which is 

 the true maximum solar intensity in the latitude of Rome. 

 It will be seen then, that, by exposing the bulb of the 

 thermometer in the manner pointed out, it is possible to 

 reduce the temperature produced by solar radiation to 

 0'38 of the actual temperature. 



It will be proper to observe with reference to the ac- 

 companying tables— constructed in accordance with the 

 result of investigations continued winter and summer 

 during three years — that the opinion expressed by the 

 Director of the Roman Observatory, respecting solar in- 

 tensity at different seasons, is wholly at variance with the 

 facts established by my numerous observations. The 

 question was raised last summer whether the high tem- 

 perature during the " heated term " would not charge the 

 atmosphere with an additional amount of vapour capable 

 of retarding the passage of the heat rays, thus rendering 

 the figures entered in my tables to some extent unreliable. 

 Accordingly, during the solstice June 26, lS7i,the sky 

 being very clear, the actinometer was put in operation for 

 the purpose of ascertaining with critical nicety whether 

 the atmosphere which had been loaded with vapour for 

 several weeks previously possessed any unusual property 

 tending to check the heating power of the sun's rays. 

 The observations were made late in the afternoon under 

 great zenith distance and increased atmospheric depth, 

 in order to subject the heat rays to an additional retar- 

 dation from the supposed vapours. The result is recorded 

 in Table C, by which it will be seen that the reduction of 

 temperature was only o'26° Fah., a difference too small 

 to call for any explanation. The result of the observa- 

 tions made during midwinter are equally conclusive with 

 reference to the permanency of solar energy at all sea- 

 sons. Among others may be mentioned that of January 17, 

 1871, the zenith distance being 6f^ 30', the actinometer 

 remained perfectly stationary at 5873° Fah., from I2h. lom. 

 to I2h. 20m. P.JI. The table just referred to shows that on 

 June 26, 1871, the actinometer indicated 53'oS" when the 

 sun's zenith distance was 61° 30'. Hence during mid- 

 winter the temperature proved to be 5873^- 53'oS° = 5'65° 

 higher, for corresponding zenith distance, than during the 

 summer solstice. By reference to Table B it will be seen 

 that owing to the diminished distance between the sun and 

 the earth, the increment of temperature on January 17, 

 ought to have been 575", discrepancy = o'i° Fah. In 

 the face of such facts it is idle to contend that the tem- 

 perature produced by solar radiation under corresponding 

 zenith distance and a clear sky, varies from any other 

 cause than the varying distance between the sun and the 

 earth. Of course there are many regions in which the 

 sun, in consequence of local peculiarities, but seldom acts 

 with maximum energy. Alaska, for instance, is hardly 

 ever favoured with a full amount of solar heat ; nor does 

 Rome,we are now informed by the Italian physicist, receive 

 maximum solar heat excepting during winter, owing, it 

 may be imagined, to the absorptive power of the atmo- 

 sphere of the Campagna during summer. 



Without entering the field of speculation, let us con- 

 sider that the established diminution of solar heat on the 

 ecliptic, nearly iS^ Fah., proves the existence of a power- 

 ful retarding medium, and points to the presence of a 

 permanent mass of aqueous matter in the higher regions 

 of the atmosphere ; necessary, it may be urged, to regu- 

 late terrestrial temperature and render vegetable life 

 possible under the destructive vicissitudes of heat and 

 cold, inevitable in the absence of a permanent regulator. 

 The assumption that the supposed mass of aqueous 



matter is nearly invariable, and at all times present, can 

 alone account satisfactorily for the remarkable fact that, 

 whenever a clear sun is presented, either by the opening 

 of the clouds or by their disappearance, the actinometer 

 indicates the same temperature, subject only to the varia- 

 tions depending on the sun's zenith distance, and the 

 varying position of the earth in its orbit. The variation 

 of temperature produced by the latter cause is entered in 

 Table B, for every fifth day in each month. This table, 

 an extract from a more elaborate one showing the tem- 

 perature for every day in the year, the meteorologist will 

 find indispensable to harmonise observations made at 

 different seasons. It may be mentioned that the attempt 

 to construct a curve, the ordinates of which would deter- 

 mine the temperature for different zenith distances, at 

 first met with apparently insuperable difficulty. The 

 result of observations made at different seasons under the 

 most favourable circumstances, failed to produce a regular 

 curve until the change of temperature corresponding with 

 the varying distance between the sun and the earth was 

 determined and introduced in the calculation. This at 

 once harmonised the previously conflicting observations, 

 and rendered the task easy of perfecting the curve, and 

 obtaining ordinates consistent with the observed tem- 

 perature produced by solar radiation at different seasons 

 and different zenith distance. 



Regarding Table A, it will suffice to state that it is 

 based upon our acquired knowledge of the temperature 

 produced by solar radiation at given zenith distances 

 when the earth is in aphelion. Evidently if we know 

 that, for instance, when the sun's zenith distance is 43^ 

 the temperature is 6o'57° Fah., we know also that this is 

 the temperature at noon on the Arctic Circle, the latter 

 being 43° from the ecliptic at the summer solstice. Again, 

 the North I'ole being 66° 30' from the ecliptic at the same 

 time, we find by referring to the figures entered in the 

 table of zenith distances and temperatures (previously 

 published) that the depth of atmosphere to be penetrated 

 by the rays when the sun is 66', 30' from the zenith, is 

 2444 times greater than on the ecliptic ; and that, therefore, 

 the radiant intensity, as shown in the table, is reduced 

 from 67-20' at the tropic of Cancer to 49'9i° Fah. at the 

 pole. Possibly it may be found necessary to introduce a 

 correction for the dillerence of atmospheric density in the 

 higher latitudes ; but at present I deem it inexpedient to 

 complicate the matter by appljing a correction which 

 obviously cannot affect the general result. 



J. Ericsson 



NOTES 



The Anniversary Meeting of the Royal Society will be held 

 on the 30th inst., when Sir Edward Sabine will c'eliver his 

 elevtntli and last anniversary address. Prof. G. B. Airy, the 

 Astronomer Royal, will be brought forward as his successor. 



At the opening meeting for the session of the Royal Geogra- 

 phical Society, on Monday evening last, the president, Sir II. 

 Rawlinson, annoraiced that, in consideration of Dr. Livingstone's 

 services, Her Majesty's Government had been pleased to grant to 

 his children the sum of 300/. 



The following are the lecture arr,angements for 1 87 1-72 at tlie 

 Royal Institution, Albemade Street :— Prof. Tyndall, F.R.S. : 

 six lectures on "Ice, Water, and Air," on December 28, 30, 

 1S71 ; January 2, 4, 6, 9, 1S72. Dr. \V. Rutherford, F.R.S.E. : 

 ten lectures on " The Nervous and Circulatory System," on 

 Tuesdays, January 16 to March 19. Prof. Odlirg, F.R.S. : ten 

 lectures on " The Chemistry of Alkalies and Alkali Manu- 

 facture," on Thursdays, January ii to March 21. Mr. W. G. 

 Clark, late Public Orator: six lectures on "The History of 

 Dramatic Literature, Ancient and Modern," on Saturdays, 

 January 20 to Feb. 24. Mr, Moncure D. Conway : four 



