OF ARTS AND SCIENCES. 157 



VIII. 



ON PORTABLE ASTRONOMICAL INSTRUMENTS AND 



THEIR USE. 



Bt Truman Henry Safford.* 



Presented, Oct. 12, 1875. 



In determining time, latitude, and the azimuth of a meridian-mark, 

 which are the principal operations of geodetic astronomy, the only 

 instrument now much used in America (besides the sextant) is the 

 portable transit, with added apparatus for speedy reversal, and a mi- 

 crometer for differences of zenith distance, according to Talcott's 

 method. A portable transit so constructed affords a speedy deter- 

 mination of all three elements mentioned above ; but its practical 

 handling is a little difficult to an astronomer used only to large fixed 

 instruments, and the determination is not always the most economical 

 in time and labor of observation and reduction. 



The present paper is intended to give a few practical hints derived 

 from actual experience with portable instruments of all grades, from 

 an ancient transit by a forgotten maker, with an object-glass which 

 would not come to a focus, up to the latest productions of the best 

 workshops of America, England, and Russia. 



The beginner will do well to practise with an instrument which is 

 not quite perfect : he thus learns in an exaggerated form all the faults 

 to which instruments are liable. 



I need not describe these instruments, but will simply refer to 

 Chauvenet's admirable Manual of Practical and Spherical Astrono-, 

 my, also to the Report of the Coast Survey for 1866. I will 

 suppose a latitude - and - longitude campaign to be planned. Tlie 

 first matter to be settled is, what instruments are to be used? If 

 the work be simply geographical, without special requirements of 

 extreme precision, like the boundaries established by the United-States 

 Land Office, and especially if the country be rough and transportation 

 very difficult, a small transit instrument will suffice. I am inclined to 



