WEATHER AND WEATHER-GLASSES. 17 



level, its average height will be about 30 inches: at an elevation of 18,000 

 feet, about 14 inches: at 36,000 feet, 3^ inches: at 36 miles, much less 

 than a hundredth of an inch. For the sake of comparison, it is usual to 

 reduce the readings of barometers to the sea -level. This may be done 

 somewhat roughly by adding one-tenth of an inch to the reading of tlie 

 barometer, for every 100 feet the place is above the sea.* The height of 

 the column is affected by many causes besides elevation. We have seen 

 tliat heat renders air lighter, and cold makes it more dense. Hence the 

 barometer is lower at the equator than in colder regions. Careful obser- 

 vation with the best instruments has shewn that the barometric column 

 rises and falls every day under the influence of the sun upon the air, quite 

 independently of storms and high winds, &c. So regular are these daily 

 tmdulations in the tropics, that the hour of the day may be known by 

 them. 



3. Corrections. 



In order to compare the barometric reading with those given m the 

 Times and Standard, it is necessary to apply a correction for elevation : — 

 Add 1-1 th of an inch for each 100 feet above the sea-level. The cor- 

 rections for temperature, &c., may be disregarded for ordinary comparison. 

 It is usual to engrave the words Change, Fair, Eain, on the dial of the 

 weather-glass. No importance should be attached to them. The obsera- 

 tions of two centm-ies show that heavy rains, and of long continuance 

 occur at Change; that rain often falls when the Mercury is at Fair; 

 and in winter, fine bright days often occur when the hand points to Eain. 

 On this subject, the following remarks which I have translated fi-om 

 Dove's ' Das Gesetz der Stiirme,' may be of service to those who watch 

 their barometers and weather-glasses. — 



" The words ' Fair,' Eain, &c , lose their value from the fact, that 

 the difference of temperature, and therefore of weight between the 

 Northerly and Southerly currents, is much greater in winter than in 



• To facilitate this simple reduction the following heights are given : — 

 Trafalgar, B.M. 662.6. 

 Rhodes Street. Lister Laiie, B.M. 600.2. 

 Haley Hill Shed, B.M. 528.3. 



Stannaiy Hall Gates, St. James's Road, B.M. 529. 

 Clare Hall, B.M. 457.6. 

 Piece Hall, B.M. 438.4. 

 Bottom of Horton Street, B.M. 408.6. 

 Top of Wade Street, B.M. 460.4. 



