SURVEYS. 



19 



rent anomaly from the Newtonian theory, — our time for observation 

 was often too limited for full series ; but this was unavoidable, as 

 the duties required of the Expidition did not admit of longer delays. 

 Notwithstanding, I hope our observations will tend to throw some 

 satisfactory light upon the subject. They are given in the chapter 

 on Tidal Observations. 



HEIGHTS. 



Heights, as connected with the surveying duties, were obtained 

 by triangulation as well as by actual levelling, observations with the 

 barometer, sympiesometer, and the thermometer, as marking the boil- 

 ing-point. They are given in a column with the tables of latitudes 

 and longitudes. 



CURRENTS. 



The velocity of the current experienced by the vessels, I have 

 inserted in a column of the table of Magnetic Variations. The dif- 

 ference between the true place and that by Dead Reckoning may be the 

 effect of current on the ship, or the result of local attraction, oftentimes 

 entirely unsuspected. Though included in the variation, but without 

 knowledge of its amount, half a point or even a few degrees of local 

 attraction would be imputed to the effect of current. It is true, there 

 is no great danger likely to arise from this omission, or want of know- 

 ledge, on a continuous route, when observations are made from day 

 to day, as it is generally allowed for in giving the course to be steered, 

 yet many serious accidents have happened to vessels from ignorance 

 of this subject. This is the reason I consider currents so intimately 

 connected with the magnetic variation to be allowed by all navigators, 

 and why I have joined the tables. I am satisfied, that one of the 

 causes why our American merchant fleet are generally so successfully 

 navigated, is owing to the absence of local attraction in our ships. 



