162 



and stands, — generally of a singularly unpractical character, — the re- 

 flecting circle is only enabled to measure inclined angles and alti- 

 tudes within very circumscribed limits. 



Any traveller, therefore, who wishes to be prepared for every op- 

 portunity for observation, has further to load himself with a theodo- 

 lite for horizontal angles, with a vertical circle for altitudes between 

 0° and 10°, and between 60° and 90°; with a transit instrument 

 for transit observations ; and with an independent telescope for ob- 

 servations of eclipses, occultations, &c. 



The author, however, by employing his particular form of the marine 

 reflecting circle, viz., the Edinburgh reflecting circle, — which is even 

 more efficient and convenient at sea than the ordinary form, — and by 

 placing it on a stand of peculiar construction, converts it at once 

 into an altitude and azimuth instrument of a most simple eff'ective 

 character ; capable of being employed as any of the above instru- 

 ments, and with some practical advantages in facility of observing 

 and reading off. 



To this combination, therefore, of the naval circle with a stand 

 for land use, he proposed to give the name of the Edinburgh Uni- 

 versal Instrument, and hoped that it might facilitate and promote 

 the observations of geographical astronomy amongst the explorers in 

 distant lands. 



2. On the Mechanical Action of Heat, Section VI. : — A re- 

 view of the Fundamental Principles of the Mechanical 

 Theory of Heat ; with remarks on the Thermic Pheno- 

 mena of Currents of Elastic Fluids, as illustrating those 

 Principles. By W. J. Macquorn Rankine, Esq. 



This section contains four sub-sections, the first three of which 

 constitute a review of the fundamental principles of the Mechanical 

 Theory of Heat, which are investigated by a method diff^erent from 

 any that has been hitherto employed ; while the fourth contains 

 the application of those principles to the determination of the infer- 

 ences to be drawn from the recent experiments of Mr Joule and 

 Prof. William Thomson on the thermic phenomena exhibited by 

 currents of air rushing through small openings. 



In the First Sub- Section, the author abstains not only from assum- 

 ing any hypothesis respecting the nature of heat, or the constitution 



