328 Mr Sang on the Determination of 



and this increase, even after the commencement of the eclipse, 

 was greater than the diminution arising from the concealment 

 of his disc by the moon ; speedily, however, the lunal obscura- 

 tion acquired the ascendancy, and the photometer rapidly sunk 

 to reach its minimum 7™ 30% after the computed time of great- 

 est obscuration. Under such unfavourable circumstances the 

 fidelity of the instrument was more than could have been antici- 

 pated, and may, perhaps, induce other meteorologists to give 

 this hitherto neglected instrument a place among their apparatus. 

 Convinced of its extreme precision, I have projected a complete 

 course of experiment for the purpose of comparing its indica- 

 tions with the degree of illumination as computed astronomically, 

 the above is the meagre substitute which circumstances have 

 compelled me to give instead. 

 \st August 1833. 



2. A Method of freeing the determination of the Latitude of an Obser- 

 vatory^ and of the Declination of a Star^ from the consideration 

 of Atmospheric Refraction. 



That element which enters most frequently into astronotiiical 

 calculations, is the latitude of the place ; next, indeed, to the 

 taking up of the meridian line, comes the determination of the 

 latitude of the observatory. Let us conceive ourselves to be de- 

 prived of all astronomical data, and to be bent on determining 

 them all anew, and we will then be able to view the determina- 

 tion of the latitude in all its bearings. 



The fixing of the transit instrument, and the regulation of 

 the clock, are easily accomplished ; tables of the differences of 

 right ascension of the fixed stars are of simple though laborious 

 construction ; but when we seek their declinations, a new and 

 complicated difficulty meets us; for, without a knowledge of 

 terrestrial refraction, we cannot determine, by any of the hitherto 

 known methods, the latitude of the place, while, without a 

 knowledge of that latitude, we cannot settle the declination of a 

 single star. And, to add to the difficulty, the refraction is sub- 

 jected to continual variations, arising from changes in the tem- 

 perature, humidity, and pressure of the atmosphere. Santini, 

 in his elaborate Ricerche sulla Latitudine dell 'Osservatorio in 

 Padova, has endeavoured to free his computation from the ele- 



