DESCRIPTION OF MAPS AND DIAGRAMS 755 



of the shading at the several points of the compass shows tlie average rise or fall of the barometer 

 per day while the wind is from those points, the + indicating a rise, and the — a fail ; the two 

 arrows starting from the centre are directed toward the points of maximum and minimum pressure ; 

 and a light line indicates the mean of the two. The arrow that springs from the circumference shows 

 the mean annual direction of the wind. In order to compensate for the rare occurrence of winds 

 from some directions, at several of the places, and make the shading more symmetrical, without 

 affecting the principle of the illustration, the mean rise or fall for each point is combined, in several 

 instances, with the two contiguous ones on either side, and the shading is proportioned to the new 

 means thus found. 



PLATE 24. 



A Meteorological Chart for Ogdensbtjrg, N. Y., 1838. 



This plate is a suggestive presentation of meteorological facts. Drawn by the author, in January, 

 1839, it is believed to be the earliest American effort to connect and vividly illustrate the mutual 

 relation between 1he results of a minute record of the winds, made by the aid of a self-registering 

 vane, and so many as five of the points chiefly noted in the registers of meteorological observers, 

 viz., amount of cloudiness, fall of rain and snow, and fluctuations in the barometer and thermometer. 

 Deductions from this chart occupy pp. 220-22Y of the Report of the Regents of the University of 

 the State of New York, for the year 1838. Each of the circles gives a synchronous view, the shading 

 corresponding in position with the wind then prevalent, and by its width indicating the amount of 

 the contrasted element. From each month, arrows radiating from the centre denote the point of 

 compass from which the wind came that was accompanied by a maximum or minimum of rainfall, 

 thermometric fluctuation, etc. 



PLATE 2 5. 



Velocity Chart. 



This illustrates minutely the general results of a series of observations, covering TOO years, and 

 taken at 418 places on the American continent, from 1854 to 1857. The object was to determine 

 what relation the average velocity of the winds, as a whole, and the varying and separate velocity 

 of each particular wind, has to the results, as to direction and prevalence, that are obtained when 

 the variation in velocity is disregarded. The solution of this question was viewed as vital to the 

 correct study of the winds, and therefore of no small importance in the search for the laws of atmo- 

 spheric circulation. 



This plate shows that the resultants computed by assigning to each wind its own separate velocity 

 differ from those in which the variation in velocity is disregarded, in being about 9" more northerly, 

 and having a magnitude of 26 instead of 23 per cent. ; and, further, that the velocity of all winds in 

 the United States, north of latitude 33°, is a little more than seven miles per hour, resulting in a 

 transfer of air in the mean direction of the main current at the rate of 2.0 or 1.7 miles per hour, 

 according as velocity is counted or omitted. 



Tlie arrows represented as flying with the atmospheric current indicate the direction of the winds 

 when only the time of their continuance is taken into account ; the dotted lines show the result when 

 the element of Velocity is also regarded. The height of the ordinates in the middle column is pro- 

 portioned to the average velocity of the wind at each season of the year. In the right-hand vertical 

 series of diagrams, the ordinates that terminate in a continuous line show the velocity of the wind 

 in the mean direction, on the supposition that the entire current moves with the foregoing average 

 velocity ; while, in contrast, those ordinates that end in the broken (dotted) linos exhibit the result, 

 as to velocity in the mean direction, when to each wind is assigned its own special velocity, when 

 the latter class oi ordinates is longer than the former, which is usually the case, the intervening space 

 contains the sign -|-. 



