110 RECORD AND DISCUSSION OP ATMOSPHERIC PRESSURE. 



In the months of December, January, and February, the variability between 

 successive diurnal means is a maximum, and in the months of June, July, and 

 August, it reaches a minimum value; the ratio of the highest and lowest being as 

 2J to 1. 



The following table contains the maxima and minima of atmospheric pressure as 

 observed in each month, and the extreme ranges for each month of the year: 



Mean range of monthly extremes 1.17 inch; the maxima rise, on the average, 

 to 30.31 inches, and the minima fall to 29.14 inches. As in the preceding table 

 of the diurnal fluctuation, the greatest monthly range takes place in December, 

 January, and February (1.48 inch), and the least in June, July, and August 

 (0.73 inch). The ratio of highest and lowest values is as 2i : 1. Between the 

 extremes, both tables show a regular progression. 



The absolute highest reading was 30.97 inches; it occurred in the morning of 

 January 22st, 1855. The absolute lowest reading was 28.84 inches, and occurred 

 near noon of February 19th, 1854. Extreme range observed 2.13 inches. 



Of the gales noted in my discussion of the winds at Van Rensselaer Harbor, 1 

 only the following ones were accompanied by a notable amount of change of atmo 

 spheric pressure : 



Before the setting in of the gale of December 28th, 1853, the barometer fell 0.35 

 inch, in nine hours, or at a rate of 0.04 inch an hour. For three days preceding 

 the gale of October 15th, 1854, the barometer was very low, and reached its lowest 

 point at the hour when the gale was at its height; after this time, it rose 0.77 inch 

 in eighteen hours, or at a rate of 0.04 inch an hour. Before the gale of December 

 18th, 1854, the barometer fell 0.3G inch in five hours, or 0.07 inch an hour; and 

 before the setting in of the gale of January 13th, 1855, it fell 0.26 inch in eleven 

 hours. 



State of tlie Barometer during the Fall of Snoiv (or Rain). To ascertain whether 

 there is any change in the barometric pressure caused by the fall of snow, I have 

 tabulated the readings during the hours of precipitation, and compared them with 

 an equal number of readings, half immediately preceding, half immediately follow- 



1 See record and results of my discussion of the observations of winds at Van Rensselaer Harbor, in 

 Yol. XI. of the Smithsonian Contributions to Knowledge. 



