88 S. A. Hill — Measurement of solar radiation, [Apeil, 



the winter months that any very great difference existed between the 

 minimum at 4 feet and that at 40 feet above the ground ; whereas in the 

 case of the maximum diurnal temperature, there was a difference of be- 

 tween V* and 2° or more throughout the year ; showing that, during the 

 heat of the day, convection must take place actively at all times of the 

 year. 



The observations at Dehra were as yet scarcely sufficient to allow of a 

 detailed comparison with those of Alipore, but it might be noticed that at 

 Calcutta the difference of the maxima at the two elevations was never so 

 small as it was at Dehra in January and February, whereas the difference 

 of the minima for an equal difference of altitude was never so great. It 

 would seem, therefore, that in the coldest months at Dehra, the nocturnal 

 cooling of the sheet of air resting immediately on the ground was so great 

 that its mean temperature for the 24 hours was 8° or 4° less than that at 

 only 70 feet above the surface. Under such circumstances there could be 

 little or no convection at any time of the day. 



3. On tlie measurement of solar radiation ly means of tTie hlach'lulh 

 tliermometer in vacuo. — By S. A. Hill, B. Sc, Meteor. Reporter to 

 Government, N.-W. Provinces and Oiidh. Commimieated hy H. F. 

 Blanfoed, F. R. S., Meteor. Reporter to tTie Government of India. 



(Abstract.) 



This consisted of a discussion of the observations of the sun thermo- 

 meter in vacuo made at the Allahabad Observatory during the years 1876- 

 1882. The same instrument had been used throughout in the same 

 position, and the observations selected for discussion were the same as in 

 the paper recently read before the Society by Mr. Blanford, viz., those of 

 cloudless or but slightly clouded days, the months July to September 

 being omitted. The author pointed out that taking as the fundamental 

 datum the difference of the Solar thermometer reading and that of the 

 maximum shaded thermometer, the values would be affected by, 1st, the 

 thickness of the atmosphere traversed ; 2nd, the absorptive power of the 

 transparent atmosphere, which chiefly depends on the water vapour pre- 

 sent ; 3rd, the haze and dust ; 4th, the radiating and reflecting powers of 

 the ground; 5th, the variable difference of the temperature at the time of 

 the maximum insolation and that shown by the shaded maximum thermo- 

 meter ; all of which must be allowed for before any conclusion could be 

 drawn as to the solar radiation intensity. A correction was applied to 

 the monthly means of the original values for the last of these variable 

 factors, and coefficients were then computed by the method of least squares 

 for the flrst and second, and monthly values being estimated for the haze, 



