42 Mr W. Galbraith's Barometric Observations. 



should be made at the same time, either by signal, or by watches 

 set together at times previously agreed upon. The instruments 

 should be protected from the sun, wind, and rain, or snow, 

 in a tent, if circumstances and convenience permit, and they 

 should be properly adjusted before making any observations. 



From my experience, I am of opinion, that the attached and 

 detached thermometers, in circumstances that permit, should be 

 allowed to come to the same temperature before any observa- 

 tions from which computations are to be made, should be re- 

 gistered, though it would be right to note them, to know whether 

 the barometer attained a state tolerably stationary. This ap- 

 pears to me necessary, for circumstances have occurred which 

 I could not explain on any other principle than that, though the 

 attached thermometer was nearly the same as the detached, yet 

 I was persuaded that the comparatively large mass of mercury 

 in the barometer tube and basin which communicated with it, 

 had not attained the same temperature, which, of course, 

 caused small variations in the height of the column, difficult to 

 be accounted for on any other principle. 



I may also remark, that when different barometers are em- 

 ployed, they should be carefully compared, to obtain their in- 

 dex errors, both at the commencement and termination of the 

 operations, to detect any alterations which might possibly have 

 taken place. On repeating the operations on different days, it 

 would be useful for the observers to change their stations alter- 

 nately from the top to the bottom. Indeed other precautions 

 will occur to the experienced observer which the circumstances 

 of the case and the nature of the situation seem to demand. 



It sometimes happens, that the observer would think it con- 

 venient to obtain the height of objects when he has not access 

 to tables or books, and in this case the sympiesometer would 

 be most convenient, since the instrument performs the whole 

 operation itself, with the exception of a simple multiplica- 

 tion. On the instruments now made, a small table is engraved, 

 from which the factor answering to the sum of the tem- 

 peratures at the top and bottom is immediately taken, by which 

 the approximate height from the sliding scale is to be multi- 

 plied to produce the true height. 



I shall endeavour here to investigate an easy formula, which 



