1890.] On the Validity of Person's Absolute Zero. 11 



requires only a quarter of the visible sky to be clear for observation, 

 and gives the value of ia directly, requiring no calculation or reduc- 

 tion. It possesses, moreover, the great advantage of not taking in 

 the low band of sky near the horizon, and thus avoiding a principal 

 element of uncertainty in other observations. 



35. The octant actinometer consists of three quadrantal planes, 

 MOS, M01, and IOS, joined at their edges so as to form a 

 hollow trihedral, and mounted so that one of the edges, OS, can be 

 brought to point to the sun ; the plane MOI will then coincide 

 with the plane of minimum intensity. The instrument has another 

 adjustment, by which it can turn round OS as an axis, and if one of 

 the planes MOS, IOS be brought to coincide with the plane of sym- 

 metry, the other will coincide with the plane of the sun's altitude. 



I take a small square of sensitised paper, and cut it along CO ; then 

 slipping the part COB under AOC, so that B coincides at C, it forms a- 

 rectangular trihedral of paper. This is placed in a small exposure 

 trihedral of cardboai-d, and covered by a thin metal trihedral in the 

 trihedral of the octant (I make several of these trihedrals of sensi- 

 tised paper, so as in the field to take quickly a series of observations ; 

 the trihedral of paper is, of course, carefully covered till the instru- 

 ment is in adjustment) ; exposed to the action of the sky for (say) 

 thirty seconds, the readings 011 the planes MOS and IOS will be each 

 30t'a, and that on the plane MOI will be 30 . \ TT i a . 



,36. Gives in Table F the observations with the octant in August 

 last. 



37. Discussion regarding the most useful method of resolution of 

 the sky and sun. 



III. " Determinations of the Heat Capacity and Heat of Fusion 

 of some Substances to test the Validity of Person's Abso- 

 lute Zero." By SPENCER UMFREVILLE PICKERING, M.A., 

 F.R.S. Received November 6, 1890. 



The relations existing between the heat of fusion of a substance 

 and its heat capacity in the liquid and solid condition were demon- 

 strated by Person, in 1847 ('Ann. Chim. Phys.' (3), vol. 21, p. 315). 

 He showed that the heat of fusion must diminish as the temperature 



