SOLAR ECLirSE, MAY «, 188;}. 61 



OBSERVATIONS OF SOLAR RADIATION. 



Ill Milditioii to tlic. oidiimry metcoroloyiciil observations, ;i scries of obsorvatious of solar 

 liuliatioii was planned. Their objeet was primarily to furnish a basis of comparison for the obscr- 

 \ations made with the same instruments durinj;; the eclipse, but they also furnish a means of diiter- 

 iiiinint;' the intensity of the solar heat on the days upon which the readinj;s were made. They arc 

 of es])ccial interest on account of the position of the observing station. 



Two kinds of instruments were employed, furuisbed through the kintbicss of the Chief Signal 

 Officer. 



1. Conjugate thermometers of the MAuii:,-L>KYY pattern. — These were made especially for the expe- 

 dition bj' Messrs. J. & H. J. Grbkn, New York, and consisted of a l)lack and bright bulb ther- 

 mometer, each inclosed iu a vacuum,* the iuclosure being spherical in shape where it surrounded 

 the bulb of the thermometer. The diameter of the iuclosure was 2.15 inches, the bulb being cen- 

 trally situate<l within it. The thermometers themselves were of the ordinary pattern with large 

 graduations on the Fahrenheit scale. Instruments of this class are sometimes made with maxi- 

 mum thermometers, but this is very iucouveuieut where readings are to be made at stated times 

 during the day. Two pairs of instruments were used, nundjcred 1, 2, 3, and 4 by the makers. 



2. Violle's conjugate Imlhs. — These consisted of two hollow copi^er spheres 4.04 inches in diam- 

 eter, one blackened, the other gilt. In eacli was placed a thermometer graduated to half degrees 

 centigrade, with its bulb blackened and located centrally within the sphere.* 



In addition to these special instruments, two ordinary thermometers, (Ikeen 113G and 1137, 

 were exposed in the sun, with the bulb of the latter blackened. The difference of the readings of 

 these thermometers is an approximate measure of the solar radiation, but the result does not have 

 the value of either of the other methods. 



The conjugate thermometers were mounted upon l>rass supports 7 inches high, and each pair 

 of supports was screwed to a board i)aiutcd black. The boards were placed upon the ground iu 

 an open grassy si>ot selected with especial care for these observations. The thermometers w ere 

 nearly horizontal, but sufficiently inclined, bulb downward, to secure an unbroken column of nier- 

 cury. The inclosures of the thermometers constituting a pair faced each other at a distance of 

 several inches, the white bulb toward the east. The board containing the instruments was turned 

 at frequent intervals during the day iu order that they might have approximately the same posi- 

 tion with regard to the sun. The Violle bulbs were mounted each upon a wooden stand 10 inches 

 above the ground, with the stems of the thermometers projecting horizontally toward the south or 

 opposite the sun. The ordinary thermometers were exposed horizontally, each attached by a brass 

 sujjport to the corresponding thermometer of the conjugate pair, and situated a couple inches 

 above it. The instruments thus placed were in a fa\'orable position for the desired observations, 

 which were made hourly or oftener upon every clear day during the stay upon Caroline Island, 

 beginning with April 27. 



*Thc instruments devised by Marid-Davy are described iu tlio Bulletiu Mensuel do I'Obs. Phys. Ceutral de Mout-' 

 souris, 1873, ]). 80, and 1874, jip. 131 aud 189. 



*A full description of these iiistrumeuts, with the formula for reducinj; the readings, is'giveu iu " Sur la Kadiation 

 Sola ire," by M. J. Viullu, Paris, 187'J. 



