Description of the Mountain Barometer. 5^ 



measured by the differences of the heights of the mercury at 

 the two stations, and these differences being evidently al- 

 ways too small in this barometer, the correction is obviously 

 always additive. As in constructing different barometers, 

 the interior and exterior diameters of the tube will not al- 

 ways be exactly similar, though the cisterns may be turned 

 always alike ; this error, and of course the correction for it^ 

 should be in each instrument deduced from a comparison 

 with a barometer of known accuracy at different heights, . 

 It will probably vary in different instruments from a nine- 

 tieth to a seventieth. Indeed, if it were always taken at an 

 eightieth, in instruments constructed as above directed, the 

 possible error could only amount to about one foot on a 

 thousand ; a quantity of very little importance. 



It now remains to say a few words o« the necessity of 

 two barometers for the mensuration of heights, and the pro- 

 bable error to be incurred by using a single one. There is 

 no doubt, that when very great accuracy is required, two 

 barometers ought to be used ; but even with every precau- 

 tion, altitudes cannot be taken by barometers sufficiently 

 near for the purpose of carrying water, either by pipes or 

 canals ; and for the purpose of the geologist, military sur- 

 veyor, or agriculturist, it is of very little importance whe- 

 ther a mountain is 1000 or 1010 feet high, though it is of 

 the highest utility that he should know whether it is 800 or 

 1000. I have, during the course oF many years, been in 

 the hai)it of taking observations of altitudes by a single ba- 

 rometer, and have had many opportunities of repeating my 

 observations on the same hills when the barometer has been 

 at different heights, and either falling or rising during the 

 time of observation ; and more than once I have observed 

 heights which had been trigonometrical ly taken by the best 

 instruments; and I can safely say that the difference be- 

 tween these observations has seldom amounicd to so much 

 as two feet on a hundred. The mode I use is this : — At 

 setting out, I take the height of the mercury, and note the 

 iinie of observation ; \ likewise note the time of the second 

 observation, and on returning to the first station, observe 



again, 

 3 



