184 Mr. Dalmahoy on the greater calorific Effect of 



In reply to the former objection it may be remarked, that 

 as x may have every magnitude from its maximum down to 

 the value of t, it follows that the ratio of a* to a* must be fre- 

 quently very large, and consequently that equation (3.) cannot 

 without error be substituted for equations (1.) and (2.). 



The answer to the other objection seems to be, that as the 

 effect of the sun's direct rays cannot be separated from the 

 effect of the 'parallel rays mingled with them, but proceeding 

 from the sky and intervening clouds, it is necessarily the united 

 intensity of these that is measured. But as the united intensity 

 of these, while the sun is above the horizon, can never be less 

 than the thermometer's radiation in the opposite direction, it 

 follows that a x can never be less than a 1 even when v is =0. 



I shall now quote one or two of the experiments on which 

 the proof of the greater calorific effect of the sun's rays in 

 high latitudes rests; and then endeavour to show that their 

 results are not at variance with the conclusions which have 

 been already deduced from the received theory of radiant 

 heat. 



In the appendix to Captain Franklin's narrative of a second 

 expedition to the shores of the polar sea, there is a valuable 

 paper by Dr. Richardson on solar radiation, from which the 

 following remarkable passage is extracted : 



" The amount of solar radiation (at Fort Franklin) shown 

 by Leslie's photometer was regularly noted in March and 

 May, and occasionally in the other months ; but when the 

 temperature of the air was low, and the sun bright, the co- 

 loured liquid was frequently driven entirely out of the limb of 

 the instrument to which the scale is attached, and in twelve 

 different instances in the month of March, the whole of the 

 liquid was forced into the colourless bulb. Whether this was 

 owing to the instrument not being calculated for measuring 

 greater solar radiation when the temperature of the air was 

 very low, or to a defect in its construction attributable to the 

 maker, I am unable to say, but as the results could only be 

 guessed at after the liquid had descended below the scale, 

 I have not inserted them in the tables. In May the liquid in 

 the photometer seldom entirely left the stem to which the 

 scale is applied, and in general in that month a degree of its 

 scale corresponded to a greater number of degrees of the 

 blackened thermometer than in March." 



It appears from this very explicit testimony that a greater 

 calorific effect of the sun's rays was frequently observed at 

 Fort Franklin in latitude 65° 12', than was ever observed 

 at London in latitude 51° 31'. But it is also worthy of re- 

 mark, that at Fort Franklin the sky was clearest in March, 



