326

CLIMATE OF NORTH AMERICA.

ture would ocoasion can be readily estimated. Underordinary circumstances, a column of 10,000 feet of airat the surface of the earth will weigh nine inches of mer-cury. Accordingly, on the morning of the 10th, a columnof 10,000 feet high of air would be 6-10ths of an inch ofmercury heavier in Iowa and on the Atlantic States than inIllinois. Or, what is the same thing, the barometer shouldstand 6-10ths of an inch lower in Illinois than in Iowa. Thisdifference is nearly equal to l-10th inch of mercury for every5° of heat. The fluctuations of the barometer from the 10thto 14th November correspond very accurately with the scaleof l-10th inch of mercury being equal to a variation of fivedegrees of temperature.

I have left out the day temperatures, and compared thefluctuations of the barometer at 7 a.m. and at 9 p.m. withthe changes of temperature at these hours in most of theStates of the Union. In order to exhibit more strikingly therelation of the temperature and the pressure of the air, thecontinuous lines in Plate 5 representing the pressure isinverted, so as to show their correspondence with the fluc-tuations of the thermometer, represented by the dotted lines.The figures on the right hand margin indicate the heights ofthe barometer in tenths of an inch; those on the left handside of the thermometer, in degrees of Fahrenheit’s scale.

By observing the fluctuations in the barometer and ther-mometer at the stations west of the Mississippi—at NewWeid in Texas, Fort Smith, Fort Snelling—we find that theyare almost simultaneous, though more than 1000 miles apart.

The changes in temperature and pressure are also simul-taneous at Montreal and North Carolina, though upwards ofnine degrees of latitude apart.

On the other hand, if the fluctuations at the stations on -the Atlantic are examined, the westerly precession of thesechanges are no less conspicuous. But, as formerly stated, ifobservation had been confined to the Atlantic, the stormwould have had an apparent progression from south-west tonorth-east, whereas the curves clearly 6how that it is nearlyfrom west to east, in conformity with the views expressed byProfessors Espy, Hare, and Loomis.