140 MEMOIRS OF THK NATIONAL ACADEMY OF SCIENCES. 



After all preliminary arraugeuieiits liad l)eeii made, I received orders from my commimdini; 

 officer, Capt.C.C.CARPENTEK,!!. S. Navy, to assume the command of the officers and men bindcd 

 to assist the Eclipse Expedition. 



I have the honor to transmit fo you herewith a repurt of the work done by me during; the 

 eclipse. 



The instrument used by me consisted, essentially, of two photoheliographs, the tnbes of which 

 were equatorially mounted on along iron pilhir or stand. Mr. Lawuance has kindly siipi>]ic<l nic 

 with the following- concise description of the apparatus. 



This instrument was constructed by J. H. Dallmeyek, of London, England, for the purpose of 

 taking photographs of the transit of Venus in 1874. The brass tube is mounted equatorially, and 

 is driven by a clock regulated by a conical governor. Tlie object glass is achromatic, with an 

 aperture of i inches. The focal length is between 5 and (! feet. The image is magnified to 4 inches 

 in diameter by a secondary magnifier placed at the principal focus of the object glass. A long 

 mahogany box, carrying at one end a rapid rectilinear lens, by Dallmeyee, with the back (M)nibi 

 nation removed, and at the other end a 3J by 4^ camera was designed to clamp to the brass tube. 

 The focus of the object glass was about 5 feet. 



Much trouble was experienced in arranging the various parts of the instrument, but the ex- 

 traordinary efiforts of Messrs. Lawbancb aud Woods were crowned with success, and by per- 

 sistent hard labor the photoheliograph was eventually set up and adjusted. 



A strong pier of bricks aud Portland cement was built aud leveled very carefully, after which 

 the iron pillar was placed upon |he foundation so obtained. This pillar was in three parts, which 

 were held together by iron bolts. 



The brass tube was fixed in place with great difficulty, as the holes in the carrying plate were 

 not in line with those in the straps encircling the tube, and it was therefore impossible to use the 

 bolts to secure it. It became necessary to resort to lashings, and, accordingly, two strong copper 

 wire seizings were passed around both tube and carrier plate, after which a strong lashing of 

 white line was added to render the fastening as rigid as possible. 



When it became desirable to affix the mahogany box to the brass tube, one of the blocks made 

 to clamp around the tube was found in two peices, aud it was necessary to secure tliis block also 

 by a lashing of white line. 



Owing to a lack of proper facilities, it was not an easy task to get the tubes parallel, and it 

 was still more difficult to retain this adjustment when once obtained. 



The method used by Mi'. LawkANCE to focus the instrument was the following: 



1. A sharp image of the cross-wire at the principal focus of the object glass is obtained by 

 moving the secondary magnifier. 



2. The object glass was screwed in or out, until the image of the sun was brought to focus. 

 The parallelism of the tubes was renewed each day up to the day of the eclipse. 



There was considerable vibration to the hing brass tube, one cause of which appeared to be a 

 lack of strength in the iron pillar, near the top, to resist torsional strain. Even the slight jar 

 occasioned by entering or withdrawing a plate gave rise to a vibratory motion, and, although the 

 trees gave great protection from the prevailing wind, a strong breeze caused the tube to tremble. 



With a view to remedy this as much as possible, guys of iieavy white line were set up to stakes 

 driven into the earth, on either side of the pillar, and these gnys were frapped together about one- 

 fourth of the distance from the top, in which manner the whole system of stays was brought very 

 taut. As an additional precaution the hishings and guys were wet about an hour before totality 

 in order to bring them as rigid as ^lossible at the time of tlie observations. 



