Jnly^, 1875] 



NATURE 



189 



exposed to the moist southerly winds, but it has two 

 advantages over the stations in the plains, viz., that there 

 are nearly 7,000 feet less atmosphere above it, and it is 

 free from the dust haze, so prevalent on the plains, which 

 perhaps more than water vapour (if not thickly con- 

 densed) stops a large part of the solar radiation. On clear 

 days and in intervals between the clouds, the sun's heat 

 is sometimes ver>' intense. The table that follows has 

 been compiled in a different manner from that which I 

 communicated a fortnight since. Instead of picking out 

 days with little or no cloud (which are sure enough 

 during the greater part of the year), I have taken the 

 three highest recorded sun-temperatures in each half- 

 month, and from these have deducted the maximum air- 

 temperatures recorded on the same days ; the mean of 

 the six observations being taken to represent the month. 

 The same instrument has been in use since the obser- 

 vations were commenced in April 1870. I must leave it 

 to meteorologists at home to compare these temperatures 

 with the recorded sun-spot areas, which I am unable to 

 ascertain. But the maximum radiation temperature evi- 

 dently falls in 1 87 1, the year of maximum spots, and the 

 increase on that of the imperfect year 1870, and the fall 

 in the subsequent years, at least up to the end of 1874, 

 arc very marked. 



Mean differences of the three highest solar temperatures in each 

 half-month and the cori-esponding maximum air tempera- 

 tures at Darjiling. 



In my former note I adverted to Prof. Koppen's re- 

 sults on the variation of the temperature of the lower 

 atmosphere in the tropics, which he showed to be inversely 

 as the number of the sun-spots or nearly so, from 1820 to 

 1858. On thinking the matter over, this result, however 

 anomalous at first sight, appears to me really only in con- 

 formity with what might be expected when taken in 

 connection with the facts of the rainfall. Since three- 

 fourths of the earth's surface are covered with water, the 

 chief effect of increased radiation must necessarily be to 

 increase the evaporation, and therefore the cloud and 

 rainfall. The former of these will intercept a larger pro- 

 portion of the solar heat and prevent its reaching the 

 ground ; while the latter, by its evaporation from the 

 land surface, will still further reduce the temperature. 

 The annual curves of temperature at the Bengal stations 

 show most strikingly how the temperature falls with cloud 

 and rain. A single heavy storm without any change in 

 the prevalent wind direction reduces the temperatuie by 

 several degrees for two or three days after the fall ; and 

 the same fact is illustrated in the mean annual curve, 

 which falls considerably on the setting in of the rains, 

 while there is generally a slight rise in September when 

 the rains draw to a close. It follows, then, that the whole 

 increase of the sun's heat and something more, in the 

 tropics, is absorbed in evaporation and by the upper 

 strata of the atmosphere, thus affording a confirmation of 

 the speculation of (I think) Sir John Herschel, that the 

 inferior planets (if partly covered by water) may enjoy an 



equable moderate temperature fitted for the existence of 

 such terrestrial organisms as can thrive under a sombre 

 sky. 

 June 7 H. F. Blanford 



SCIENCE IN GERMANY 

 {From a German Corrrspondent.) 

 "D UNSEN'S ice-calorimeter was used lately for a very 

 ^-^ interesting experimLnt by Messrs. Rontgen and 

 Exner, who tried to determine the intensity of the radia- 

 tion of the sun by means of an apparatus constructed on 

 the principle of that calorimeter. The apparatus consists 

 of a glass bell a of 75 mm. height. This is fastened into 

 a brass hoop b, which is closed below by a plate of 

 wrought silver of j mm. thickness, and 106 mm. diameter. 

 The neck of the bell bears a massive brass top d, which 

 is cut conically above and below, and has a central 

 opening of 6 mm. diameter. Into the exterior groove a 

 massive brass cone e fits water-tight, having also the 

 central boring, into which a little glass tube is fastened. 

 By a screw h in the circumference, the cone e can be 

 firmly pressed against the brass piece d, while the tube/ 

 communicates with the interior of the bell a. A second 

 communication between the interior of the bell and the 



outside is obtained by the boring at g and the metal-tube 

 /, with stopcock /. 



When the apparatus is to be used as a pyrheliomcter, 

 the bell is filled with well-boiled distilled water, and 

 the whole is frozen like one of Bunsen's calorimeters. To 

 the tube/ along glass tube of perfect caUbre and with 

 millimetre divisions is fastened by means of a piece of 

 india-rubber tubing ; to the end /• of the brass tube with 

 stopcock an indiarubber ball filled with well-boiled 

 water is then fastened, the stopcock opened, and while 

 the apparatus is held vertically, all air which may still be 

 contained in the bell is removed from it through the cone 

 ^', the tube /, and the divided tube, so that thse latter is 

 filled with water up to its end. Then the stopcock / is 

 closed. If beforehand the silver plate has been carefully 

 covered with soot, the apparatus is ready for use. It is 

 directed towards the sun just.like Pouillet's pyrheliometer, 

 so that the sun's rays fall vertically upon the blackened 

 plate. The divided tube is then supported as much as 

 possible in a horizontal position, and the progress of the 

 column of water in the same is observed with a second 

 clock from minute to minute. This progress of the 



