Page:The American Cyclopædia (1879) Volume XI.djvu/459

 METEOROLOGY 445 systematic variations in the barometric pressure can be best shown graphically, and it will suf- fice to present only a few typical cases. The existence of a perceptible lunar tide in the atmosphere is now no longer considered prob- able, and the important periodic changes all depend upon the sun. The diurnal changes, as shown in diagram XIII. for Nertchinsk, Siberia, and Plymouth, England, illustrate respectively the continental and maritime stations. The ef- fect of elevation above the surrounding country (not merely above sea level) is seen in the con- trasts between the diurnal changes at Geneva (altitude 1,335 ft.) and at the summit of Mt. St. Bernard (8,114 ft.), as shown in diagram XIV., both for January and July. At the sea level, two maxima and two minima of pressure occur during each 24 hours ; the maxima be- The full lines are for St. Bernard ; dotted lines for Geneva. DIAGRAM XIV. Diurnal Variations in the Barometer. tween 9 and 11 A. M. and between 9 and 11 P. M., the minima between 3 and 5 P. M. and between 2 and 5 A. M., respectively. The an- nual barometric changes are shown -in diagram XY. as given by Lorenz and Rothe (1874) for typical stations, whence the great influence of geographical position is easily seen. A clear- er perception of the important part played by aqueous vapor will be obtained if from the whole atmospheric pressure shown in diagrams XIII., XIV., and XV. we subtract that portion due to aqueous vapor, as shown in diagrams IX. and X. ; the remainder is technically known as the pressure of the dry air, or the gaseous atmosphere. It is found that the gaseous and the vaporous components have each a single diurnal and annual fluctuation, and that it is the combination of these that produces the irregular and even double fluctuations in the total pressure shown in the preceding dia- grams. A general view of the annual baromet- ric changes, and a more complete insight into the relation between pressure,, wind, and weath- Eeykiavik. Camlridge, Enf. Toronto. DIAGRAM XV. Annual Variations in Atmospheric Pressure. er, are obtained by the comparison of a series of charts, such as VII. and VIII., which show for the respective months the average distribu- tion of the pressure of the atmosphere over the surface of the globe. The variations of baro- metric pressure with the latitude were first defi-