38 



king 127 in all, not including registers from several stations too remote lo 

 be of any service in the proposed investigation. 



I commenced the analysis of the storm of February 1-5, by following 

 the same course I had pursued with the storm of 183(3, viz: representing 

 the barometric observations by curves which should exhibit the fall and 

 rise of the mercury, with the time of minimum height; and from the time 

 of minima I proposed to deduce the progress of the storm. But I was 

 disappointed in my expectations. I did not obtain such a uniform rate of 

 Progress as I had anticipated. After some time I perceived that my observa- 

 tions embraced two centres of disturbance; that I had got at least two 

 storms in close juxtaposition, and interfering with each other. I then dis- 

 covered that my lines of barometric mininnmi represented relations which 

 were extremely complex, and were not well adapted to my purpose, which 

 was the development of physical causes. 1 therefore sought for some mode 

 of graphically representing the observations which should be founded upon 

 simple relations, and be better adapted to suggest the causes of the phenom- 

 ena. I at last settled upon a method which appears to me well suited to this 

 purpose, and substituted for lines oi minimum, pressure, lines of equal pres- 

 sure. Having determined, as well as I was able, the mean height of the 

 barometer at each station, I compared each observation with the mean. I 

 then drew a line upon a map of the United States, passing through all 

 those places where the barometer stood at its mean height. This line 

 may be called the line 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 6 inches, 8 inches, and 100 inches below the mean height. 

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

 the centre of these curves must be a point of maximum 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 tor 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- 



