248 Mr. Dalton on Meteorology. [April, 



been about two-tenths of an inch. It was not till after another 

 period of five years that I determined to renew the barometer. 

 A tube was taken of about one-seventh of an inch internal 

 diameter, having a large bulb at the lower extremity ; this was 

 filled with mercury that had a few minutes before been boiled in 

 order to expel the air and moisture more effectually. This was 

 found to stand nearly two-tenths of an inch higher than the 

 previous one, and a few hundredths higher than it stood at ori- 

 ginally ; owing in part at least to the elevation or height of the 

 barometer, above the level of the sea, being 10 or 12 feet more 

 in the first period than in the latter. In order to allow for the 

 rise and fall of the mercury in the bulb, the scale of four inches 

 was made only 3*98 inches, and subdivided into tenths as usual. 

 The observations throughout the whole series were taken three 

 times each day; namely, at 8 a. pi. and at 1 and 11 p. m. The 

 new or last mentioned barometer has been used for the last 15 

 years, commencing with January, 1804 ; and it may be right to 

 observe, that for the three last months of 1798 it was used, but 

 the reduction was applied to the monthly means, in order to 

 make the observarions of the whole year uniform. As the adhe- 

 sion of the mercury to the tube is more or less observable in 

 great variations, I make it a practice to give one or two gentle 

 vibrations to the mercury prior to any observation, w hich 1 think 

 is more accurate than using a wider tube without such vibratiolis. 



The elevation of my barometer above the level of the sea is 

 nearly 180 feet ; it is about 100 feet above the level of the Duke 

 of Bridgewater's canal, and that is nearly 80 feet above the sea. 

 This elevation is rather greater than would be deduced from the 

 observations of the barometer for the last period of 15 years, in 

 which the annual average height of the instrument is about 

 29-91. On the supposition that the average height of the baro- 

 meter on a level with the sea is 30 inches, my barometer w^ould 

 be inferred to be about 90 feet above the sea ; but I conceive 

 the real average height of the barometer on a level with the sea 

 has never been accurately determined. Possibly there may be 

 some difference in the specific gravity of the mercury used for 

 this instrument. 



During so long a period of years, there must have occurred 

 interruptions to the observations, but I never delegated any 

 person to supply for me ; these interruptions, however, scarcely 

 happened, except in the month of July, when they have been 

 very frequent for half of the month. These blanks, however, have 

 been filled up partly from the observatiqas of Mr. Thomas Han- 

 son, of this town, w^ho has kept a meteorological journal with 

 great attention for several years, and partly from the observa^ 

 tions of the Royal Society and other authentic sources, due 

 allowance being always made for the difference in the absolute 

 heights of the two barometers. 



