08 MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



1. ItEDUCTION OF OBSEItTATIONti MADE WJTII THE CONJUGATE TEEItMOMETEBS. 



The bulletins of the Central Physical Observatory of Montsoiiris contain a general description 

 of tlie instruments, but no rigid investigation of tbeir theory. The differences between the read- 

 ings of the black and bright therniometers are taken as a measure of the sun's intensity, and are 

 used in the formula T — t — dp', in which T', t are the readiug-s of the therniometers respectively, 

 fi the solar constant, jj the diathermancy constant, and e the thickness of the layers of atmospheric 

 air traversed by the solar rays. It is recognized, however, that the values of obtained by this 

 formula are not the true solar constant, but vary with the iustrnments used,* having a value of 

 17° in the instruments used at Moutsonris. For the purposes of comparison of the observations 

 made from time to time, it is snggested that a conventional value of 100° be adopted for H, and 

 that observations with different instruments be reduced to this standai'd by multiplying by a 

 factor depending upon the value of H for each instrument. At Montsouris, where 6 = 17° for the 

 ])nir of instruments described, the factor is Yt"- = ^-SS, and the product of the observed differences 

 T' — t by this factor is called the "actimometric degree." In the case of observations made in 

 1873 and 1874 the ditt'erences 1' — t are published, but in observations since that time the acti- 

 mometric degree is computed. 



Quite recently a rigid investigation of the theory of these instruments has been made by 

 Prof. William Fekrkl, and is to be published in Professional Paper of the United States Sig- 

 nal Service No. XIII. It assumes the law of Diilong and Petit, and derives formula? for 

 deducing the solar intensities and also the solar constant. Through the courtesy of the author, 

 the formula; have been furnished for this computation in advance of their publication. They are 

 as follows: 



" Let ti = the temperature of the black bulb. 

 ^'i = the temperature of the bright bulb. 

 II' — the temperature of the inclosure (shade temp, approx.). 

 I = the intensity of solar radiation (calorics per minute on square centimetre). 



n = constant 1.0077 (log tx — ., ^^ 



1 



ui 



= constant to be determined from observation. 



We have then the following relations: 

 (1) I = 4.584 (yu»— ;u»') = 4.584 /^e'(;^»-e' — l) 



(2) I = 4.584 (//« — |<''')c= 4.584 /('" (yi^e_s._l)c 



(3) //e. = P/<''.+ (l-c)/A" 



From (1) the intensity is obtained from the readings of the black bulb in vacuo and the air 

 temperatui'e, which is approximately equal to that of the inclosure, or from (2) the intensity is 

 obtained from the readings of the black and bright bulbs in vacuo, the constant c being obtained 

 from (3). The method assumes that c can be obtained from observation, and thus the readings of 

 the black and bright bulbs in vacuo be used in computing the solar intensities; 0, o„ and o' are 

 expressed in centigrade degrees. 



* Bull. Mens, de I'Obs. Phys. Centr. de Moutsomis, 1874, p. 189. 



