INTRODUCTION. 



MAGNETICAL OBSERVATIONS, 18491852. 



In the programme submitted to and approved by the joint committee of the American Phi- 

 losophical Society and the American Academy of Arts and Sciences it was proposed that the As- 

 tronomical Expedition should employ a portion of its time as follows : 



" Magnetical Observations. On one day of every month the term-day of such observatories 

 as continue to keep it I propose to determine the three elements of the earth's magnetic con- 

 dition, viz : The declination, inclination, and the intensity of the directive force. To verify the 

 law of diurnal fluctuations of the angle which the directive force makes with the true meridian, 

 observations with the declinometer will, occasionally, be continued throughout the term-day. 

 Neither the appropriation granted by Congress, nor the aid asked for, will justify any extension 

 of these observations, but, if suggested, modifications of times or methods will be cheerfully con- 

 formed to. 



" Meteorological Observations. A register will of course be kept, and it is proposed that it 

 shall embrace a record of the indications of the barometer ; dry, wet, radiating, and registering 

 thermometers ; wind-vane ; clouds and rain-gauge the records to be made at 9 A. M. and 3 and 

 9 P. M. , or such other hours as will furnish results from which the mean and extremes of the 

 climate of Chile may be more satisfactorily derived. To determine fully the local law of daily 

 variations, it is in contemplation to make hourly observations throughout the equinoctial and 

 solstitial term-days, as long as the expedition may continue in Chile." 



It will be seen in the following pages that the number of observations of both classes was 

 more than doubled. Believing that magnetic observations, at short intervals and with similarly 

 constructed instruments, might lead to most interesting results, it was proposed by Professor A. 

 D. BACHE, the distinguished physicist charged with the United States Coast Survey, that we 

 should record the indications of the declinometer from IV/i. to ISA., and of the horizontal force 

 magnetometer from 18/i. to 197t. Gottingen mean time on the first day of each month, unless 

 that day fell on Sunday, and then during the same hours on the second day of the month. The 

 observations were to be made as rapidly as possible, say at intervals equal to the times of vibra- 

 tion of the bars, or some multiple of those times. Professor Bache intended to cause similar 

 observations, by at least one party, on each coast of the United States ; and it was presumed 

 that both the observatory at Toronto, (Canada,) and the officers of the Commission sent to de- 

 termine in conjunction with one from Mexico the boundary between that country and the United 

 States, would also co-operate with us. 



