506 Mr. J. J. Watcrston on Solar Radiation. 



greater than in the other. In the same way we might compute 

 the potential temperature of an angular space occupied by many 

 thousand flames placed one behind the other, extending in a line 

 from the observer, and probably we should find it cumulative in 

 the ratio of the number of flames. 



From observations I have made on gas-flames with the radia- 

 tion-meter, fig. 1, it would seem to require about 4000 batV 

 wing flames ranged behind each other to give a potential equal 

 to that of the sun. 



If the upper radiating matter of the sun is in any degree 

 transparent or permeable to radiation from lower strata, it is 

 obvious that the actual temperature may thus be much below 

 the potential. 



26 Royal Crescent, Edinburgh, 

 November 25, 1861. 



EXPLANATION OF THE FIGURES IN PLATE V. 



Fig. 1. T U B E is a square tube of brass, mounted with motion in 

 altitude upon an upright, R, fixed into a round slab of lead. The inner 

 surface of this tube is blackened, and at each end, at I and c, a film of 

 transparent talc was stuck on to prevent the wind from moving the air 

 within the tube. 



H D, D H a double screen made of cardboard and cork, coated on both 

 sides with tinfoil, and fitted to slip on the extremity of the tube presented 

 towards the sun. 



m, the hole in the centre of screen, about T Vth inch greater diameter 

 than the bulb of the solar thermometer, X. 



The talc film I was also coated with tinfoil, except the central circle. 



X, the thermometer in sun with spherical bulb fixed in a cork that fitted 

 the hole L, L in top of brass tube. 



Y, the thermometer in the shade fixed in the hole N, N, with cork and 

 soft wax as shown. 



Z, a thermometer applied to outer surface of tube. 



Fig. 2 is a transverse section of vacuum-bath, employed to ascertain the 

 rate of cooling of the solar thermometer X in air and in vacuo. 



It consists of a cylindrical vessel of brass, coated internally with lamp- 

 black ; the lid L is ground to the upper edge of the cylinder, and in its 

 centre is a stuffing-box, S, with Indian-rubber collar, through which the 

 stem of the thermometer is passed, as shown in the figure ; C is a stopcock, 

 upon which N, the nozzle of a flexible tube communicating with an air- 

 pump, is ground air-tight. H is a wooden handle for removing the appa- 

 ratus to and from the water-bath without touching the metal. 



Fig. 3 is a Chart of the observations taken in July 28, August 6 and 21 . 

 Each observation is represented by a point, the ordinate of which is the 

 reciprocal of the radiation potential, and the abscissa of which is the secant 

 of the apparent zenith distance of the sun's centre. 



Fig. 4 is a duplex scale, showing the correction to be applied to the radia- 

 tion potential as observed in air to what it would be if in vacuo. It con- 

 forms to the observations ou cooling in air and in vacuo given in the Ap- 

 pendix, and was computed from the empirical formula therein described. 



The correction in degrees and tenths is found on the right-hand side of 

 the line opposite the value of ra entered on the left-hand side. It has to 

 be added to r„. 



