ABOUT THE 20th OF DECEMBER, 1836. l29 



successively at distant stations, we call one wave, understanding, of course, 

 that the same wave, at successive instants, may be formed by entirely 

 different particles of water. The same atmospheric wave, then, passed over 

 the continent from west to east, from the valley of the Mississippi to the 

 Bermudas and Nev/foundland. We wish now to be able to assign the 

 rate of progress of this wave, and in order to institute the comparison, we 

 must fix upon some particular plmse of the wave. We might make the com- 

 parison for the times of either the barometric maximum or minimum. The 

 latter, however, is much the best suited to our purpose, as the barometer 

 remained much longer near the high than the low extreme. There is a diffi- 

 culty, however, in ascertaining the precise instant of even the barometric mini- 

 mum. When observations are taken but two or three times a day, it is highly 

 improbable that the minimum should occur at the precise instant of either ob- 

 servation. There may, therefore, be an uncertainty in the time to the amount 

 of twelve hours, or more. Fortunately, however, this uncertainty is, in a mea- 

 sure, obviated in the present instance. The hourly observations made at Bal- 

 timore, Montreal, Albany, Flushing, New Haven and Gardiner, exhibit the 

 precise form of the barometric curve described, and the instant of greatest de- 

 pression. Moreover, as will appear in the sequel, this minimum was contem- 

 poraneous with a remarkable change of wind, and the time of this change was 

 noted at several stations; as, for example, at Springfield, Syracuse, Philadel- 

 phia, New York, and Hanover. At nearl}/ half of the stations, then, the time 

 of minimum is certainly known; and the form of the curve at these stations 

 will guide us in completing the curve at the remaining stations. This conjec- 

 tural completion of the curves is represented on the chart by dotted lines. We 

 are then enabled to trace the progress of this wave with very considerable pre- 

 cision. I accordingly drew upon a map of the United States linos connecting 

 all those places where the barometer attained its minimum at the same instant. 

 The result is shown on the accompanying chart, which exhibits the lines of 

 greatest depression for every six hours from the morning of December 20th to 

 the noon of the 23d. The velocity of the wave appears not to have been uni- 

 form. Thus, on the southern border of the United States its velocity varied 

 from seventeen to twenty-nine statute miles per hour; and on the northern bor- 

 ders from seventeen to thirty-seven miles per hour. No allowance is here 



VII. — 2 H 



