.548 MULL. METEOROLOGY. BAROMETER. 



instrument by subjecting it to many thousand inversions, 

 as being that motion to which it is subjected by use ; but 

 without effect. 



I shall now enumerate a few of the most remarkable 

 differences observed on distant barometers. 



Between London and Greenwich* the differences were 

 found to vary from to 0,1, 0,2, 0,25, 0,28, in quantity; 

 while the times of the greatest elevation or depression also 

 occasionally varied from twelve to twenty-four hours ; 

 the one barometer sometimes reaching the lowest or 

 highest point first, and sometimes the other. Similar 

 differences, but less in quantity, were found to take place 

 between my own house and the Royal Observatory, distant 

 about a mile ; but these, it will soon appear, were chiefly 

 the results of the errors of construction, within the limits of 

 which they were found to lie : they rarely exceeded a few 

 hundredths of an inch. Between Perth and Edinburgh, 

 similar differences, but much greater in quantity and in 

 time, were found to exist ; and, as in the first case, they 

 were most conspicuous when the mercury was undergoing 

 rapid and extreme changes. The differences in this exam- 

 ple varied from to 0,2, 0,26, 0,3, 0,5, 0,57 ; the actual 

 difference of level in the two stations being sixty-four feet, 

 not equal to 0,1 of the barometric column. I may here 

 remark that it is not very unusual to estimate altitudes 

 by this method, when the horizontal distances of the 

 instruments have been even greater than this. Between 

 Greenwich and Edinburgh, as might be expected, still 

 greater differences were observed ; ranging from to 0,4, 

 0,55, 0,6, 0,63 ; and in this instance it was remarkable that 

 the Greenwich barometer sometimes occupied the lowest 

 station, contrary to the general rule respecting latitude. 

 The difference of level in this case is only twenty-one feet. 

 I do not think* it necessary to detail more of these obser- 



* I must here observe that in all these comparisons the differences of 

 level between the stations were kept in 'view and allowed for. 



