420 Capt. Luetke's Account of E.rperiments •joith an 



may be made a lliermometer which will indicate by its change 

 of tint very slight changes of temperature. The temperature 

 at which a ray of definite refrangibility has the optic axes 

 corresponding to it united, so as to form a single system of 

 rings, is a point as well fixed as that of boiling water, and every 

 different inclination of the optic axes of definite rays indicates 

 two different temperatures in the scale of heat, equidistant 

 from those other points at which the same rays have their axes 

 united. 



The accurate measurement of these angles would no doubt 

 be difficult, but an instrument might be made to show them 

 by inspection. The temperatures, however, might be more 

 simply indicated by the great variety of tints successively de- 

 veloped by heat; and as each tint has a numerical value in 

 the scale of colours, its accuracy would not be much less than 

 that of the other method. 



Allerly, Nov. 3rd, 1832. 



LXXII. An Account of Experiments with an Invariable 

 Pendulum, during a Russian Scientijic Voyage. By Captain 



LUETKE*. 



'T'HE observations of the invariable pendulum occupied the 

 -■■ first place amongst the scientific researches to which our 

 attention was directed during the circumnavigation of the 

 Shiiavine. All these observations are ali-eady calculated : but, 

 as 1 have not yet been able to give them the form in which 

 they will ultimately appear before the public, and as a new 

 appointment confines me at present to other duties, I trust 

 that a summary account of these observations and their re- 

 sults will be acceptable to the Imperial Academy of Sciences, 

 as well as to the scientific world in general. 



The apparatus, which we matle use of in these experiments, 

 is the same as that which had been previously adopted by 

 Capt. Basil Hall, at the several stations in South America. It 

 is, in fact, the same in construction as that which was em- 

 ployed by Capt. Sabine in his voyage to Spitzbergen. Before 

 quitting England, a series of experiments was made at the 

 Observatory at Greenwich: and again on our return. The 

 second series gave a result, which differed from the first, about 

 ■/(^tlis of a vibration, in excess; which I attribute to a slight 

 wearing of the knife edge. This difference ought perhaps to 

 be distributed over the whole interval, in arithmetical pro- 



* Translated from tlie BuUclin Scicnlijique, page xi., attached to the 

 Memoirs of the Imp. Acad, of Sciences at 8t. Petersburgh. Series vi. vol. i. 

 (IS.-iO). 



