1858.] On Hypsometrical Measurements. 353 



11. "With respect to the method of making observations with the 

 boiling-point thermometer, it is only necessary to observe that the 

 instrument described by Professor Forbes in the paper already re- 

 ferred to # seems the most convenient and trustworthy of any that 

 has been proposed, and very superior to that described by Colonel 

 Sykes,t and still more so to that manufactured by Casella and sold 

 in India along with Prinsep's Table — an instrument which never 

 could be expected to give accurate results. Professor Forbes's 

 boiling apparatus consisted of a thin copper pan heated by a " Ens- 

 eian Furnace," having a powerful jet of inflamed alcoholic vapour, 

 which might be removed to one side until the escape of steam be- 

 came uniform and moderate, and could be used in a gale of wind. 

 The thermometer had its bulb immersed in the water, of which a 

 moderately large quantity is requisite to a good result. 



The following examples will shew the use of the tables and for- 

 mulae. 



Example I. The following data are given in " Smith and Thuil- 

 lier's Manual of Surveying" (p. 436) — Boiling-point at the lower 

 station 208°. 7 and temperature of the air 83° ; at the upper station 

 B. P. 204°.2, and air 75° Faht., to find the difference of elevation, 

 By the formula (18)— 



D = 212 — 20S.7 = 3.3. D' = 212 — 2042 = 7.8. 

 And DD' = 25.7. T + T' = 412.9. T — T' = 4.5. 

 And J (t + f) = 79°. 



Now 167.32 + 412.9 =580.22 



25.7 



-h = 0.04 



600 ■ 



580.26. ar. co. log 7.23638 — 10. 



T — T' = 4.5 log 0.65321. 



§ (t + tO = 79° log A 4.82175. 



Const, log 0.70827. 



Height 2628 feet log 3.41961. 



* Edin. Trans, vol. xv. 



t See Journal of the Royal Geographical Society, vol. viii. p. 436- 



