Mr W. Galbraitirs Barometric Observations. 41 



servation, and some other minor considerations, for purposes 

 of extreme accuracy, as far as could by that method be obtained. 

 Indeed there is no method, where great precision is required, 

 that does not involve difficulties and sources of error in a greater 

 or less degree. The variable nature of terrestrial refraction, 

 the distance of the objects observed from the situation of the 

 instruments in trigonometrical surveying on a grand scale, as 

 that of the British Islands by the Board of Ordnance, all tend 

 to involve circumstances that produce error. The barometric 

 method is not, therefoi more liable to error than the geome- 

 trical when performed trigonomelrically, even with the best in- 

 struments, which are much more expensive, while the method of 

 levelling, in the usual acceptation of the term, is seldom prac- 

 ticable for considerable heights. In the barometric method, 

 the instruments are of moderate expense, especially the sym- 

 piesometer ; and it will be seen, that, so far as my experience 

 extends, the results derived from it are nearly of equal accu- 

 racy with those of the best mountain barometers, while its cheap- 

 ness and portability are additional recommendations. It ap- 

 pears to me, however, that the sympiesometer requires rather 

 more care in its use than the mountain barometer, on account 

 of its being more rapidly susceptible of receiving impressions 

 from a change of pressure from currents of air, from changes of 

 temperature, and even, I am not sure that it does not occasion- 

 ally suffer changes difficult to be accounted for from photome- 

 trical influence. All these circumstances require the observer 

 to bestow great care in the use of it, so as to prevent any irre- 

 gular influence to affect it unequally during the time of obser- 

 vation, on account of its extreme susceptibility. Though the 

 mercurial barometer is a less suddenly susceptible instrument, 

 yet the same care, nearly, must be employed when it is used, 

 otherwise accurate results will not be obtained. Hasty obser- 

 vations should never be made, except from necessity, or where 

 great precision is not required. I have repeatedly found, that 

 when the observations are made too hurriedly, little confidence 

 can be placed in them. To procure the utmost accuracy, good 

 weather should be chosen, when the barometer is steady ; two 

 barometers should be used, one at the top of the height to be 

 measured, and the other at the bottom, and the observations 



