148 



NATURE 



[June 2^, 1875 



It is evident that India offers far greater advantages 

 for investigating the variations of the solar heat than 

 any European country can do, and as observations of the 

 black-bulb thermometer m vacuo have now been registered 

 at several stations during the last six or seven years, 

 I have lately examined a portion of these, to see if they 

 afford any direct evidence of a periodical graduated varia- 

 tion in the intensity of the radiation. The result is to me 

 very striking, and if not absolutely conclusive as to the 

 direct variation of the sun's heat with the number of 

 the spots and prominences, certainly, as far as it goes, 

 strongly confirms Mr. Baxendell's conclusions, drawn 

 from indirect evidence. 



It is unfortunate that owing to the fragility of the 

 instruments employed and the necessity of exposing them 

 freely, they are very frequently broken ; and, as a conse- 

 quence, the longest series of observations made with one 

 and the same instrument extends over only five years. 

 This is at Silchar in Eastern Bengal. The place is 

 situated in lat. 25°, therefore beyond the tropic ; and the 

 climate being very damp and more cloudy than most 

 parts of Bengal, it is not, perhaps, so favourably circum- 

 stanced for the present purpose as some other stations. 



The means of the maximum sun-temperatures registered 

 on clear days (that is, on days when the proportion of 

 clear sky estimated at 10 a.m. and 4 P.M. did not average 

 less than three-fifths) are given in the following table. 

 The months of the S.W. monsoon are omitted, since in 

 some cases they do not furnish a single clear day accor- 

 ding to the above definition, and as a rule such days are 

 too rare to contribute much evidence of value. I give 

 for each month the number of clear days that have contri- 

 buted to the mean. 



Table I. — Average maximum temperature of solar radiation on 

 clear days at Silchar. 



Did this table stand alone, the evidence of any periodical 

 variation would be very doubtful. But we shall presently 

 see that the irregularities that it exhibits are all but com- 

 pletely neutralised by the registers of other stations. 

 It is easy to suggest their explanation, grounded on the 

 fact to which all the registers testify, that the highest sun- 

 temperatures occur, not on days registered as cloudless, 

 but on those on which there is a considerable proportion 

 of cloud, and frequently rain. Such days were numerous 

 in 1874 ; while in 1871 (the year of sun-spot maximum) 

 days without visible cloud predominated. Leaving the 

 discussion of this question, however, as unnecessary in 

 this place, I will give the combined results of Silchar and 

 eight other Observatories variously situated, some in, and 

 others beyond the tropical zone. These are :— 



Port Blair, in the Andamans lat. 11° 41' N. 



Cuttack, in Orissa ... ... ... , 



Chittagong, on the Arakan coast ... , 



Jessore, on the Gangetic delta ... , 



Dacca, also on the delta ... ... , 



Hazaribagb, * elev. 2,000 ft. in Western Bengal , 

 Berhampore,* on the Gangetic delta ... 

 Roorkee, elev. 900 ft. in the N.W. Prov. 

 Since the radiation- thermometers originally i 



* The registers of these two stations taken alone give a curve nearly 

 approximating to the resultant of all the stations, but it is of doubtful 

 validity o-wing to the thermometers having been twice renewed at both 

 stations. 



most of these stations have been broken and replaced by 

 other instruments, and since these thermometers (fur- 

 nished by the best London makers) sometimes differ to 

 the extent of many degrees when placed under the same 

 conditions of exposure, it would be only misleading to 

 compare together the registers of different years recorded 

 with different instruments at the same station. In order 

 to avoid this source of error, and at the same time to 

 bring in evidence as much as possible of the registers, I 

 have taken for each station separately the difference (rise 

 or fall indicated respectively by + and - ) of each pair of 

 homonymous months in consecutive years, omitting all 

 cases in which the instrument has been changed in the 

 interval ; and then the mean of all the differences thus 

 obtained for the same pair of months. The results are 

 given in the following table, additional columns being 

 added to show how many stations have contributed to the 

 mean of each pair of months. As in Table I., the mean 

 temperatures compared are those of clear days only ; but 

 with the exception of Port Blair, I have admitted as 

 clear days those only on which at least four- fifths of the 

 sky on an average was estimated as unclouded at 10 a.m. 

 and 4 P.M. In the case of Port Blair it was necessary to 

 admit days with only one half of unclouded sky.* 



Table II. — Annual variation of mean maximum readings of 

 black-bulb thermometers on clear days. 



If these differences be plotted as the increments of a 

 series of ordinates. and the curve thus marked out be 

 corrected for its small irregularities libera vianu, its 

 resemblance in general character to the sun-spot curve 

 will be distinctly apparent. (See figure.) 



I have been unable to ascertain (here in Calcutta) the 

 number of spots observed during the last few years ; but 

 this datum can readily be suppUed at home. 



Calcutta, May 28 Henry F. Blanford 



LECTURES AT THE ZOOLOGICAL GARDENS^ 

 VIII. 



Mr. Sclater on the Pheasants. 

 T N that Birds possess a high temperature of the blood, 

 ■*■ they agree more with the mammalian than with other 

 vertebrated animals ; the balance of anatomical evidence 



" * I ha-ve ascertained by direct comparison that any difference thus intro- 

 duced is inappreciable, the results beinj; treated comparatively, and not for 

 absolute values. 



t Continued from p. 130. 



