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those places where the harometer stood at its mean height. This Une 

 may be called the Une of mean pressure. I then drew a line through all 

 the places where the barometer stood 2 inches above the mean, and another 

 for 4 inches above the mean. So, also, I drew a line through all the places 

 where the barometer stood 2 inches below the mean ; another for 4 inches be- 

 low; and others for G inches, S inches, and 1 00 inches behnv the moan height. 

 These lines must, from the nature of the case, be continuous curves; and 

 the centre of these curves must be a point of maxinuun or minimum 

 pressure. Near the middle of a great storm we find a point of minimum 

 pressure — that is, the centre of a vortex; and the lines of equal pressure 

 will indicate at a glance whatever connexion there may be between the 

 weight of the air and the direction of the wind. 



Upon a similar principle, all the observations of the thermometer were 

 graphically represented upon the same chart. A line joining all those 

 places where the thermometer stands at its mean height for the given hour 

 and month is marked zero, and may be called the line of mean tempera- 

 ture. Another line joins all those places where the thermometer is 10° 

 above the mean, and others for 20° and 30° above the mean. So, also, 

 lines are drawn for 10° and 20° below the mean temperature. There are 

 also continuous lines surrounding a point of maximum or minimum ther- 

 mometric disturbance; not points of maximum or mininumi temperature 

 absolutely, for we only regard the deviation from the mean temperature of 

 the place for the given hour and month. These barometric and ther- 

 mometric curves exhibit some conformity to each other, but are far from 

 being identical. The direction of the wind is represented by an-ows, and 

 its force is indicated, so far as convenient, by their length.* 



Other phenomena are now indicated by colors. Those regions where 

 the sky was unclouded, or where the cloudiness was less than one- half, 

 are colored blue; those where the sky was entirely overcast, or the cloudi- 

 ness exceeded one-half, but without rain or snow, are colored brown. 

 Those regions upon which snow is falling, are colored green; and those 

 where rain is falling, are colored yellow. Thus, nearly every important 

 circumstance of a storm is presented to the eye at a single glance. All 

 these particulars will be understood from the two accompanying charts, 

 illustrating the progress of the storm of February 16, 1842. 



In both the storms of February, 1842, after they had acquired consider- 

 able violence, there was a prevalent motion of the winds iiiward, with a 

 tendency to circulate around the centre, in a direction contrary to the sun's 

 motion. The motion of the wind, therefore, was neither wholly centrip- 

 etal nor wholly rotary, but a combination of the two. As long as the 

 wind was moderate, neither of these tendencies M^as clearly marked, and 

 there were numerous perplexing anomalies, probably occasioned in many 

 instances by the inequalities of the earth's surface. The storm of Febru- 

 ary 16 travelled in one day 560 miles in a direction N. 53° E., making its 

 velocity 23 miles per hour. The storm of February 1-5 remained for 

 two days nearly stationary, and then travelled N. 62° E. at the rate of 36 

 miles per hour. On the whole, then, we may conclude that when storms 

 are violent, and there is a great depression of the barometer, the direction 

 of the wind presents considerable regularity, being spirally inward towards 



*Por an account of the method employed by Mr. Espy in representing the phases of storma, 

 see his communication, appendix JNo. 3. 



