PORTABLE BAROMETER. || 



barometers for the mensuration of heights, and the probable sessoonsidera- 

 . 11- -1 rpi • 1 w '^^'^ accuracy, 



error to be mcnrred by using a single one. 1 here is no doubt, 



That where very great accuracy is required, two barometers 

 ought to be ujed ; but even with every precaution, altitudes 

 cannot be taken by barometers sufficiently near for the purpose 

 of carrying water either by pipes or canals, and for the pur- 

 pose of the geologist, militar}' surveyor, or agriculturist, it is 

 of very little importance whether 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 

 yea^^ been in the habit of taking observations of altitudes by a 

 single barometer, and have had many opportunities of repeat- 

 ing my observations on the same hills when the barometer has 

 been at different heights, and when falling or rising during the 

 time of observation ; and more than once I have observed 

 heights which had been trigonomically taken by the best in- 

 struments, and I can safely say that the difference between 

 these observations have seldom amounted to so much as two 

 feet on an hundred. The mode I use is this; — At setting out ^cronnt of tlie 

 I take the height of the mercury, and note the time of obser- "/jled bVtIi* " 

 vation. I hkewise note the time of the second observation, author. 

 and on returning to the first statjoe, observe again and note the 

 time. Jfthe barometer has altered in the interval, a simple 

 proportion corrects cither of the three observations, and redu- 

 ces the height to what would have been observed had the mer- 

 cury been stationary. It is true, that this method supposes 

 the motion of the mercury to have been uniform during the 

 interval of obsei'vation, but except in very variable \veather, 

 A\ hich does not very often occur, particularly in summer, wi.cn. 

 the greater number of these observations will naturally be 

 made, this sup])osition may be safely made. It is also 

 true, that a traveller has often no opportunity of making a 

 ■second observation at the spot he set out from. Even in this 

 case, a near approximation may often be made by observ- 

 ing, for example, at a stream on each side of the hill to 

 be measured. If also he observes the barometer repeatedly in 

 the morning before he sets out, and sees its tendency, and does 

 . the same at every halt during the day, he will have data where- 

 on to found a nearly accurate correction. But if all this <^'''''"e""atioi)s 



. -^ are higlily use- 



should be out of his power, even under the most unfavourable ful, even nnder 



^, . unfavourable 



