294 Dr. Apjohn on Wollast oris Thermometer. [Oct. 



its temperature increases in an arithmetic ratio, nor would it, 

 thus calculated, have been materially less accurate. To illus- 

 trate its application, we will here give the particulars of a 

 measurement lately made with au instrument belonging to Dr. 

 Stokes. This instrument was made by Mr. Lovez, of Edinburgh. 

 Its bulb is l-{- inch in diameter, and it possesses a scale of 

 13-16ths of an inch to a degree. The degree is divided into 20 

 equal parts on an attached brass scale, and these are further 

 divisible into l-20ths by means of a sliding vernier. To this 

 short description, I will merely add, that the point 212 was fixed 

 for a pressure of 30, referring for a more detailed account of the 

 mode of constructing similar instruments to those papers of Dr. 

 Wollaston's already alluded to. Near the gate leading to Dr. 

 Stokes's house at Ballantyne, the instrument, in the steam of 

 boiling water, stood steadily at 21 1 + 4&» the thermometer at the 

 same time indicating a temperature of 68*5 ; and on the top of 

 the Three Rock Mountain, we found that the boiling point had 

 descended to 209 -f- 4$, the thermometer having likewise fallen 

 to 62. By a reference to the table, then, it will appear that the 

 entire ascent consists of v/^ths of 535*692 together with 533-352 

 and -^ths of 531*006 feet, making a sum total of 1170*199 feet. 

 The correction for temperature is applied in the following 

 manner : Let the tabular height 1170*199 be multiplied by 33*2 

 (the excess of the arithmetic mean of the temperatures of the 

 two stations above 32), and let the product be multiplied by 208 

 (which we will call the constant multiplier), and the decimal point 

 be moved five places to the left, the resulting number 80*809, 

 added to the tabular weight, will give 1251*008 feet for the true 

 height of the Three Rock Mountain over Ballyntyne. It will be 

 here observed that I do not use the same correction with Dr. 

 Wollaston. His is greater, and was adopted on the authority of 

 General Roy. Mine is taken from a work on Mechanics, pub- 

 lished by the Rev. Mr. Robinson, late Fellow of Dublin College, 

 and is that which occurs in the barometric formula of Laplace. 

 As has been already stated, the instrument on the Three Rock 

 Mountain stood at 209 + -i£, therm. 62 ; but in about half an 

 hour after, on Garrycastle, an adjacent mountain, it fell to 209 

 -f J-g-, therm. 62 still. It is manifest, then, that the tabular 

 height is half of 535*692, which, when corrected in the manner 

 already specified, gives for the height of Garrycastle over the 

 ThreeRock Mountain 284*559 feet. By adding this to 1251*000 

 feet, the true height of the Three Rock Mountain over Ballyn- 

 tyne, we shall have for the elevation of Garrycastle over Ballyn- 

 tyne 1535*561 feet. Now the heights of these mountains, as 

 determined barometrically by Dr. Htton and Mr. Weaver (see 

 vol. v. of the Geolog. Trans. Memoir by Thomas Weaver, Esq.) 

 are: Three Rock, 1247*9; Garrycastle, 15317, which numbers 

 scarcely differ from those given above. The above results are 

 Tery satisfactory, and indeed their close correspondence with the> 



