204 On the Decomposition of 



executed by Russian officers, accompanied by one or more of 

 the French officers, who have been engaged in the operations 

 which we have described. 



It is obvious, however, that even for the deductions to be 

 drawn from the arcs of the parallel, the extension and multi- 

 plication of the measurements on the meridian are already 

 become more requisite than of those on the parallel. 



We merely avail ourselves of the occasion to express a hope, 

 that before the operations on either parallel are considered to 

 be concluded, the endeavour may have been made to obtain 

 the celestial amplitude of each, by astronomical observations, 

 executed in concert at the extreme stations. For arcs of small 

 extent, there cannot be a doubt that the determination by sig- 

 nals is far preferable to every other method ; but there must be 

 a term in the extension of the distance to be measured, when 

 the celestial reference from the extremities equals in value, and 

 in still greater extensions must be regarded as preferable to the 

 method by signals. We are not prepared to say which method 

 might deserve the preference on arcs of the magnitude of those 

 of Marennes and Orsowa, Brest and Czernowitz ; because we 

 could assign by conjecture only the degree of accuracy attain- 

 able in several months of diligent and concerted observation, to 

 determine the difference of meridians of the extreme stations, 

 by all possible phenomena, particularly by lunar transits and 

 occupations. But we cannot doubt that the attempt, which 

 would be far less costly than the operation by the other method, 

 would lead to highly valuable conclusions, not only in producing 

 just notions of the relative value of the methods ; but in fur- 

 nishing a result, which, in comparison with that obtained by the 

 instantaneous signals, could not fail to be most advantageous and 

 important to the arcs themselves. 



E. S. 



On the Probable Decomposition of certain Gaseous Com- 

 pounds of Carbon and Hydrogen, during sudden expansion. 



Some very singular appearances have been observed by Mr. 

 Gordon, of the Portable Gas Works, which have led him to 

 believe- that chemical changes are occasioned by the sudden 



