74 



I made use of a position micrometer, devised for the purpose oi' 

 rapidly determining their places on the sun's limb. A circular plate 

 of metal, 8 inches in diameter, was attached, by a collar passing 

 through its centre, to the sliding tube of the telescope, to which it 

 was firmly clamped, so as not to turn round. This plate was 

 covered with a disc of card on the side next the eye-end of the 

 telescope. Inside the tube carrying the plate, another tube carrying 

 the eye-piece, slid smoothly, so as to admit of being freely turned 

 round. The latter tube was furnished with two springy arms, point- 

 ing in opposite directions, in front of the plate, like the hands of a 

 clock, and having steel points, by which holes could be pricked in 

 the card disc. A small level was attached at right angles to one of 

 these arms, and parallel to the card disc. In the eye-piece were 

 fixed three equidistant parallel spider lines, the outer two being 

 placed at an interval equal to the moon's apparent diameter calcu- 

 lated for the time of the total phase of the eclipse ; so that when the 

 outer wires were made to embrace the moon's disc, the middle wire 

 would pass through its centre. The instrument was adjusted for 

 observation, by causing the middle wire to coincide with a plumb- 

 line, seen at a distance through the telescope ; while, at the same 

 time, the bubble of the level was brought to the middle of its tube 

 by turning the arms, which were then securely clamped to the tube 

 carrying the eye-piece. It is evident that if, after this adjustment, 

 the bubble were again brought to the middle of the tube, while the 

 outer wires were made to embrace the sun's disc, the middle wire 

 would pass through its vertex ; and two holes being pricked in the 

 card, the line joining them would represent the sun's vertical diameter 

 at the time of observation. If next, while the sun was kept between 

 the outer wires, the middle wire was made to bisect any object at its 

 limb, and holes were again pricked in the card, the angles between 

 the lines joining the respective pairs of holes would measure the 

 angular distance of the object from the sun's vertex. In this manner 

 the positions of the red prominences, seen during the total phase of 

 the eclipse, could be rapidly registered on the card, without ever re- 

 moving the eye from the telescope. 



The observations of time were made by means of a box chrono- 

 meter by Adams of London, obligingly furnished by Lieutenant C. 

 A. Pettersson, of the Navigation School of Gbteborg. It was com- 

 pared with his standard chronometer about 3^ 15™ before the com- 



