REDUCING TO THE TRUE MEAN BAROMETRIC PRESSURE. 



431 



When the Barometrical means to be used have been derived from observations 

 taken at sucli hours of the day as, if combined, do not give the true mean pressure, 

 they must be reduced to the true means by using the Tables XX. and XXT. 

 These tables give the corrections to be applied to the hourly means, in each month, 

 for reducing them to the means which would have been given by observations made 

 at each of the twenty-four iiours. The correction for any given set of hours is found 

 by taking the mean of the corrections due to each of the combined hours, paying 

 due attention to the signs. Table XX. has been computed from the hourly obser- 

 vations made under the superintendence of Professor A. D. Bache, at Girard College, 

 Philadelphia. Table XXI. is from the Greenwich Observations, by Glaisher. 



XX. 



North America. — Philadelphia. Lat. 39° 58' N. Long. 75° IP W. Greenw. 



Corrections to be applied to the Means of the Hours of Observation 1o obtain the true 

 Mean Barometric Pressure of the respective Days, Months, and of the Year. 



Barometer in English Inches. 



D 



91 



