THERMOMETER AND PYROMETER. 



43 



Fig. 41. 



the socket is made to unscrew from the 

 sole of the instrument. The ball, b, is 

 of black, or deep reddish-brown enamel, 

 while c is as diaphanous as possible. 

 The graduation, and other parts of the 

 photometer, are on the same scale and 

 construction as in the differential ther- 

 mometer. 



Dr. Franklin, and others, had re- 

 marked the superior power which dark 

 colours possessed of absorbing the ca- 

 lorific influence of the sun's rays ; and 

 Dr. Watson, afterwards Bishop of Llan- 

 daff, had, in 1773, observed, that when 

 a thermometer, having its ball black- 

 ened, was exposed to the sun's light, it 

 rose 1 higher than it had previously 

 done in a similar situation. The re- 

 searches of Mr. Leslie put this fact in a 

 more striking point of view, and led to 

 the invention of this instrument. 



The theory of the photometer hangs 

 on the supposition, that the intensity of 

 lii;ht emitted from any body, is always 



Eroportional to the temperature excited 

 y its incidence on the blackened ball. 

 This is probably true with regard to the 

 undecomposed rays of the sun, in which 

 the caloric and the light, if different 



kinds of matter, are intimately blended ; 

 but there is strong reason to suspect, that 

 light emitted by terrestrial bodies is not 

 always proportional to the concomitant 

 temperature. Thus the intense splen- 

 dour of phosphorus burning in oxy- 

 gen gas, gives out far less heat than the 

 comparatively dull combustion of hy- 

 drogen in the same gas ; and we have 

 found this photometer often more af- 

 fected by the emanations from a fire so 

 duD, that not a single letter could be 

 discerned in a well-printed page, than 

 by the degree of daylight, by which we 

 could read the same print with plea- 

 sure and facility. It is differently af- 

 fected too by light of different colours, 

 where their illuminating property ap- 

 pears the same ; and the experiments of 

 Herschel, Englefield, and others, show 

 that the maximum of heat in the solar 

 beam, decomposed by the prism, by no 

 means corresponds with the illumina- 

 tion, but is even altogether beyond the 

 margin of the spectrum.* 



As a measure, however, of the inten- 

 sity of undecomposed solar light, it ap- 

 pears to support the character it re- 

 ceives from the inventor. " The pho- 

 tometer," says he, " exhibits distinctly 

 the progress of illumination from the 

 morning's dawn to the full vigour of 

 noon, and thence its gradual decline 

 till evening spreads her sober mantle. 

 It marks the growth of light from the 

 winter solstice to the height of sum- 

 mer, and its subsequent decay through 

 the dusky shades of autumn ; and 

 also enables us to compare, with nu- 

 merical accuracy, the brightness of 

 different countries the brilliant sky 

 of Italy, for instance, with the murky 

 air of Holland." 



The direct impression of the sun's 

 rays at noon, about the summer sol- 

 stice, in this country, equals from 90 

 to 100 of this instrument ; and at mid- 

 winter, the force of the solar beams is 

 from 25 to 28. The indirect light, 

 from a summer's sky, at noon, is from 

 30 to 40; in winter, it is from 10 to 

 1 5. In the most gloomy weather,, in 

 summer, the photometer rarely indicates 

 less than 10 at noon ; but in winter it 

 sometimes barely exceeds a single de- 

 gree. 



The observations on the light of day 

 with this instrument should always be 

 made in the open air ; and the direct 



* Herschel, Phil. Trans. 1800; Englefield, Journ. 

 Roy. Institution, vol. i. 



