Description of the Mount am ^arimeier . i;^ 



do not exist ; and in., those parts where a knowledge of the 

 comparative heights of the difterent hills is the most gene- 

 rally useful, they seldom exceed 1000 feet ; at all events, 

 such observations as relate to great elevations may be alvyjLye 

 recomputed. by more rigorous methods at leisure. 



The correction of the heights thus obtained, for the tepi- 

 perature,of,.th9 air above freezing, is by sir George Shuck- 

 burgh SLipposed to be as the height of the thermometer, and 

 to be 2-44 thousandth of the approximate height for each 

 degree of Fahrenheit, additive when the temperature is above 

 freezing, and subtractive when below freezing. General 

 Roy's, observations and experiments lead to a supposition 

 tl^>t|.^h9;icp;:Tection is not exactly as the height of the ther- 

 mometer, and that at about the temperature of 50 degrees 

 it amounts to 2*3 — thousandths, and is less, both much 

 above and much below that temperature. For the purpose 

 of immediate computation, I take the correction at 2*5,- 

 which, though certainly rather too great, will in general be 

 productive of very small error, and affords a rule which is 

 easily remembered and quickly applied. It is this ; 



For every four degrees that the mean temperature of the 

 two detached thermometers exceeds 32 degrees, add one 

 hundredth of the approximate height, as before obtained, 

 to it ; for every 40 degrees one tenth, and so for any greater 

 or lesser number of degrees. * , 



I have not hitherto mentioned the correction, which i-n 

 fact ought to be the first in order, viz. that i'or the differ- 

 ence of temperature of the- t^vo baromelers themselves : but 

 this cbfrection is in general so small, ^s"' to be safely neg- 

 lected. Should it, howev^er, be thought necessary to apply 

 it in this approximate method of coniputing heights, the 

 rule deduced^ from sir George Shuckbui^gh's mtfthod'^r'^^ate 



follows, and it wants no'-t4ble, thou^^h be "h^^ gl'i^eti^'8& 

 ^•qj. j^^ \ ■ /HfiMiO'' . U id tk:v'. li^*' ■ ■'■ •■ ■. 



3 ''When the barometer slihds at 2i[> inche^'^ tfier expansrdh 

 of the mercury for one degree of Fahrenheit is two-thou- 

 sandth of aTi inch, when it stands at 30 inches it is three- 

 thousandths, and for the intermediate inches it increases (f^- 



D 4 actly 



