338 Mr Galbraith on Trigonometrical Surveying 



Now, allowing 8.5 feet for the half tide, as before, there 

 will be obtained from this series 



8.5 feet. 

 175.0 ... 

 4.0 ... 

 1701.5 ... 



1889.0 ... 

 1880.7 .. 



8.3 ... 



Half the rise of tide, 



Height of ground at station, . 



Height of circle, .... 



Height of Carnethy above circle, 



Total height, .... 



Barometric height verified by levelling, 



Excess of Trigonometrical height, 



These differences of 8 and 11 feet only serve to shew that, 

 by the new method of computing the co- efficient of refraction 

 from the state of the barometer and thermometer, both more 

 exact and more consistent results are obtained than by using 

 a mean value generally, especially upon the summits of dis- 

 tant high mountains, where the pressure and temperature dif- 

 fer considerably from the mean. 



It is long since a table of mean refractions has been rejected 

 in astronomy where any attention is paid to accuracy, and 

 after this example, it is to be hoped that the actual co-efficient 

 of terrestrial refraction will be computed for pressure and 

 temperature uniformly in the same manner as the astrono- 

 mical refractions generally are. It was with this view that I 

 have transformed my equation so as to be enabled to employ 

 the same tables for both. Indeed, if the mean of our present 

 deductions be taken, the first series gives . 1869.4 feet. 

 The second series, . . . 1889.0 ... 



Of which the mean is, 1879.2 . . . 



agreeing almost exactly with the barometric measurement, 

 confirmed by Mr Jardine's levelling, though the difference 

 between the two amounts to 19.6 feet, a difference likely 

 owing to small unavoidable errors of observation and atmo- 

 spheric irregularities which no care and address can obviate. 

 From this conclusion, it also appears that all the three me- 

 thods concur in bringing out the same conclusion when per- 

 formed with equal care, though the same result could not be 

 expected when any of them is hastily executed. 



It has been known to several individuals for some years 



