XX ORIGIN AND OPERATIONS 



derivable from a comparison of observations made at observatories in opposite hemispheres 

 -th niul *>uth) on the circum-zenith stars of each other, each observatory to determine also 

 the absolute places of the same stars. I therefore propose to observe most carefully the declina- 

 tions of Columbw, ,9 Columba), 6 Centauri and / Scorpii, and the zenith distances of /9 Persei, 



i . - H.Tcnlis, Lyric, aud 67 Cygni. 



h. Observations of comets. For the reason assigned with reference to observations after 

 1 A. M., it cannot be expected that we shall devote much time to searches for these bodies, how- 

 ever desirable it might be under the serene atmosphere of Chile. They will, however, be 

 observed whenever discovered in the course of other observations or at periodic returns. 



Ttli. Magnetic observations. On one day of every month the t<-r>n-<l<iy of such observatories 

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

 dition, viz: ita 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 exten- 

 sion of these observations ; but, if suggested, modifications of times or methods will be cheer- 

 fully conformed to. 



"8th. Meteorological observations. A register will of course be kept, and it is proposed that 

 it shall embrace a record of the indications of 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 U 

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

 Chilean climate may be more satisfactorily derived. To determine the local law of daily varia- 

 tions fully, it is in contemplation to make hourly observations on equinoctial and solsticial term- 

 days during our whole residence. 



" 9th. There is one other phenomenon connected with the physical constitution of our globe 

 which the residence of the Expedition may offer opportunity to investigate. I mean the terrible 

 convulsions that its crust is in many places subject to, and to which the very locality we shall 

 occupy has been so frequently and so fatally liable earthquakes ! It is therefore submitted, 

 whether any and what form of seismometer shall constitute a part of our equipment. 



"These nine classes or series of observations embrace as great an amount of labor as it will be 

 prudent for two observers to undertake, and even ite accomplishment must of necessity leave all 

 reductions until alter the return of the Expedition to the United States ; but (whilst I disclaim 

 knowledge of nearly all branches of natural history) as so little has been learned of the imme- 

 diate country we shall probably select if the collection of specimens at leisure hours, remarks 

 concerning the flowering of plants, the migration of birds, or other designated phenomena, 

 would be useful from one so unskilled the enterprise is embarked upon with a full determina- 

 tion to gather every scientific fruit that may be offered. 



"The various observations actually required, and those which it appears proper to make, hav- 

 ing each been enumerated, I proceed to specify the instruments absolutely necessary for the 

 purpose. 



//. A meridian circle. [Any construction of this instrument which will facilitate observa- 

 tions without impairing accuracy will be of the utmost consequence in an establishment so 

 feebly manned. I therefore present for consideration that the circle be] at least 36 inches in 

 diameter, reading to 1" by means of four micrometer microscopes, and to be provided with a 

 telescope of not less than 52 lines clear aperture. [The telescope to be constructed with a pris- 

 matic reflector at the centre of its transverse axis, so placed that rays of light from the object- 

 glass will 1 ! through the axis as a part of the telescope, and thus the observer will 

 occupy tli" Minn- position whatever may be the altitude of the observed object. At the same 

 tiin.', tli.- tfli-scopi- tube is to be made symmetrical, in order that the eye-tube, with its micro- 

 nn-trrf, nmy IK- placed a in the ordinary construction, should it be found i-: \prdient to remove 

 the prism. A disc of metal or of wood, 3'J inches in diameter, interposed between the observer 



