652 HISTORY OF SCIENCE, 



is indicated by the point of the ivory pin a just touching the surface of 

 the mercury. As the height of the mercurial column varies with the 

 temperature, it is always necessary in exact observations to make such 

 a correction as would give the height at some defined temperature, 

 usually o C. A thermometer c is therefore attached to the barometer. 



Barometric observations were long carried on, and the results re- 

 gistered at many stations, and these were compared with the state of 

 the wind and weather at each place. Certain rules were sometimes 

 deduced in which it was attempted to establish a relation between the 

 barometer reading and the forthcoming weather at some of the localities. 

 The reliance which could be placed upon these rules was, however, 

 usually very small. When the more accurately constructed barometers 

 had come into use, it was soon found that, independently of all other 

 variation, the barometer has certain regular diurnal oscillations. It 

 rises from 4 a.m. to 10 a.m., falls again until 4 p.m., rises between 

 4 p.m. and 10 p.m., and falls from the last-named hour to 4 a.m. In 

 tropical countries these variations are so regular that the hour of the 

 day might be determined from the maxima and minima readings within 

 a possible error not exceeding the third part of an hour. An annual 

 variation of the barometer has also been noted. Many interesting 

 results of this kind have been obtained by self-registering instruments. 

 Some of these are highly ingenious contrivances, giving a continuous 

 record of the pressure of the atmosphere. Thus in the barograph used 

 at Kew Observatory and other British meteorological stations, the 

 height of the mercury is marked on sensitive photographic paper moved 

 by clockwork night and day. At Greenwich the height of the column 

 of mercury, exhibits on the average, variations the range of which may 

 be thus indicated : over 132 days in the year the change is less than 

 o't inch, on 123 days it lies between o - i and 0*2, on 61 days between 

 o'2 and 0*3, on 27 days between o'3 and 0*4, on 12 days between 0-4 

 and 0*5, on 6 days between o'5 and o - 6, and on 4 days between o'6 

 and i inch. Not, perhaps, oftener than once in ten years will it happen 

 that the change of the barometer amounts to 1*25 inch within twenty- 

 four hours. 



While barometric observations were conducted independently, that 

 is, without synchronous comparison, the hourly, daily, monthly, and 

 yearly averages at each station were all the results the observations 

 could show. A new era of meteorological progress was inaugurated 

 with the system of comparing the barometrical readings taken simul- 

 taneously at different places, reported by telegraph, and collected 

 together. But while barometers were still inaccurate in their con- 

 struction, many investigators were comparing recorded results and 

 reducing them to systematic expressions and actual laws : we might 

 mention Sir John Ross, Sir John Franklin, and many others. At length 

 Dove broached his celebrated gyratory theory of storms. According 

 to this theory, in all great storms there is a wind blowing round an 



