448 



The Astronomer Royal on the 



[Apr. 27, 



exclusively and without any loss to warm the products, is such that an 

 augmentation of pressure always takes place at a constant volume, or, 

 what is the same thing, an augmentation of volume at a constant pressure. 



This proposition results not from any a priori deduction, but is verified 

 by the whole of facts known to this day. 



7. One may ask if the change of volume, in which the gases keep the 

 whole heat produced by their mutual actions, is regulated by a simple law, 

 ^analogous to those that have been observed when the gaseous combinations 

 are brought to the same temperature ; nevertheless it does not appear to 

 be so. 



Let us compare the formation of the different hydracids by means of 

 their gaseous elements, which gives no change of volume when the gas is 

 reduced at 0° and m 760. 



The formation of chlorhydric gas, H CI, produces 23,900 calories ; the 

 formation of bromhydric gas, H Br, produces 13,400 calories; the for- 

 mation of iodhydric gas, HI, produces 800 calories. The specific heat of 

 these gases being nearly the same under the same volume, it is clear that 

 the quantities of heat aforesaid cannot produce an augmentation of volumes 

 identical or proportional with simple numbers. 



II. i( Remarks on the Determination of a Ship's Place at Sea." In 

 a Letter to Prof. Stokes. By G. B. Airy, LL.D. } &c, Astro- 

 nomer Royal. 



Eoyal Observatory, Greenwich, S.E., 

 1871, April 5. 



My dear Sir, — In the last published Number of Proceedings of the 

 Royal Society*, there are remarks by Sir William Thomson on the pro- 

 posed method for determining the locus of a ship's place at sea, by making 

 one observation of the sun's (or other body's) altitude, and founding, on 

 this, computations of longitude with two assumptions of latitude ; and there 

 are suggestions, with a specimen of tables, for solving the spherical tri- 

 angles which occur in all similar nautical observations, on the principle of 

 drawing a perpendicular arc of great circle from one angle of a spherical 

 triangle upon the opposite side. 



In regard to this principle and the tables which may be used with it, I 

 may call attention to the employment of a similar method by Major- 

 General Shortrede, in his ' Latitude and Declination Tables,' pp. 148 and 

 180. In p. 150, line 1 1 from the bottom, it will be seen that the "column" 

 gives the trial-value of the perpendicular arc by which the two right-angled 

 triangles are computed. This is not the same (among the various elements 

 which may be chosen) as Sir William Thomson's ; but it is so closely re- 

 lated that in some instances the tabular numbers are identically the same 

 as Sir W. Thomson's, though in a different order. General Shortrede's 



* Vol. xix. p. 259. 



