REDUCING TO THE TRUE MEAN BAROMETRIC PRESSURE. 



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



taken at such 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 XXI. 



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 hours. 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 



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



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



'hiladelphia. Table XXI. is from the Greenwich Observations, by Glaisher. 



XX. 



NORTH AMERICA. PHILADELPHIA. Lat. 39 58' N. Long. 75 11' W. Greenw. 



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

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



Barometer in English Inches. 



D 



93 



