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 he 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 1ST. 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 1, fig. 5,) and those applied to the San Diego observations were taken from Table No. 2. 
SEA LEVEL, f 
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 pressure 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 64° 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 
* This has no practical value except as an approximate base from which was determined the measure of abnormal error 
for each hour in the twenty-four. 
f See plate 11. 
