BAROMETRIC PRESSURE AT SEA LEVEL. 5 



drawn connecting these observations. The observations at the Presidio de San Francisco, made 

 under the direction of Lieutenant W. P. Trowbridge, United States Engineers, of the Coast 

 Survey, and permitted to be used for this comparison by Captain W. R. Palmer, United 

 States Topographical Engineers, in charge of that office, are plotted in their relative positions, 

 as are the various observations taken during the survey, and also those on Colorado river, 

 taken under the direction of Lieutenant N. Michler, United States Topographical Engineers, 

 of the Mexican Boundary Commission, and kindly offered to be used in this discussion by 

 Major W. H. Emory, commissioner. The horary correction was not eliminated from most of 

 these observations, which will account for the minor irregularities observed in the several curves. 

 When the combination for the scale of correction was made all the proper corrections were 

 applied. The mean line expressed in the plates is about the mean of the Benicia and San 

 Diego observations, combined, viz: 29.650.* The horizontal scale of the diagrams is one-tenth 

 of an inch to three hours, and the vertical scale one inch to one inch of the barometer. On 

 Plate 5 the arrows represent the direction of the wind and the state of the weather at the specified 

 hours, during a remarkable storm on the 1st and 4th of January, 1855. 



It is not proposed to go into a discussion of this principle of compensation for abnormal effects, 

 or to draw any general conclusions from the results obtained ; the facts are simply stated, and this 

 task is left to the able and zealous meteorologists who are now engaged in tracing the finger that 

 points the storm. 



To the observations at Benicia the horary corrections applied were deduced from the long 

 series of hourly observation of Lieutenant Trowbridge, taken at the Presidio de San Francisco, 

 (see Plate I, fig. 5,) and those applied to the San Diego observations were taken from Table No. 2. 



SEA LEVEL, t 



The mean of nine months observation at Benicia barracks, 81.5 feet above tide, with all 

 the corrections applied, including correction due to 81.5 feet, gave for the mean jiressure of 

 sea level 30.057. At San Jose, at the southern extremity of the bay of San Francisco, the 

 estimated height of that place above tide, applied to the observations taken there, gave as a 

 base 30.040. This base was used as far as the middle waters of the Salinas, (Camp 8.) 

 The mean of thirteen observations on the beach in the bay of San Luis Obispo, with an 

 average air temperature of 63°, gave a result of 30.031. (See general table.) The mean of 

 eight observations at the sea level between the Gaviote pass and San Buenaventura, with a 

 mean air temperature of 68°, gave for the sea level east of Point Concepcion 30.023, and a 

 single observation with air temperature of 04° gave 30.022, and another at 65°. 2 gave 30.023. 

 To the above observations it should be born in mind that all the corrections were first applied, 

 including the abnormal error, as determined by the method above referred to. There are 

 many other points along the coast, situated from 10 to 40 feet above mean tide, which are cor- 

 rectly fixed by the barometic observations. From these results it appears that the maximum 

 pressure for the Pacific coast attains a higher latitude than on the Atlantic, and will probably 

 be found, by subsequent observations, to be in about latitude 43'^ north, or at some point 



• Tliis has no practical value except as an appro.ximate base from wliich was determined tlio measure of abnormal error 

 for each hour in the twenty-four. 



f See plate XI. 



