Mr Galbraith's Barometric Measurements of Heights. 329 



Without due caution, therefore, these unavoidable errors aris- 

 ing from single sets of observations made after the lapse of some 

 hours, when the barometer is changing somewhat rapidly, may 

 easily, though improperly, be imputed to the faulty construc- 

 tion of the instruments employed instead of the proper source. 



Lastly, I may remark, that it is of importance, for the sake 

 of accuracy too, that the lengths of the mercurial columns should 

 be correctly reduced to the same standard temperature which, in 

 a continued series of observations, is generally taken at the freez- 

 ing point, or 32° of Fahrenheit's scale. I have therefore given 

 concise tables for that purpose in a former Number of this Jour- 

 nal, for 1828, because such reductions are often inaccurately 

 made by allowing the expansion of mercury in glass, instead of 

 the absolute expansion of mercury in barometers of the'common 

 construction. In mountain barometers, the expansion of the 

 brass tube enclosing the glass one, on which brass tube the scale 

 for measuring the height is engraved, must also be taken into 

 account, which in these tables has likewise been attended to. 



To enable observers to make a proper and accurate allowance 

 for capillary action, I have thei-e likewise given a table, computed 

 from Mr Ivory's formula, to every hundredth of an inch, be- 

 cause by the irregular variation, interpolation for intermediate 

 points cannot be accurately made by even proportion. I have 

 been induced to notice this here, because in books of some re- 

 putation, and in cotemporary Journals, these reductions are made 

 erroneously, — ^by that means forcibly bringing about coinci- 

 dences of apparent accuracy, when no such thing could be said 

 either of the instruments or the observations. 



EXPLANATION OP THE TABLES. 



Table I. The numbers in this table are derived from a table 

 of logarithms adapted particularly to barometric observations. 

 It is on the same principles as that of Oltmanns"', in English feet, 

 with proportional parts annexed. Since the numbers are given 

 to inches and tenths, the proportional parts for hundredths are 

 conveniently placed on the left, opposite the tenths. 



The same proportional parts answer for thousandths, by strik- 

 ing off a figure from the right, increasing the last or units figure 

 remaining by 1, if the figure struck off' exceeds 5. Thus in all 



JULY SEPTIiMliEK 1831. V 



