358 SURVEY OF THE COAST 



die d, which can be moved by its milled head, and presses 

 with its lower part against tlie cord part of the same piece 

 below; the upper screws in the bridge thus forcing its middle 

 up or relaxing it by the mere spring of the metal. This is 

 all the adjustment which is allowed to the supports, as it is 

 supposed that the stone pillars, and therefore the plates, may 

 be brought within these limits by previous levelling, and 

 thereby greater stability be obtained. 



About five inches from both ends of the axis, are two 

 strong rings, of about four inches diameter, turned exactly 

 on their edges. These turn upon perpendicular rollers of 

 the same diameter, which are pressed against them from be- 

 low, instead of counterpoises, by means of springs enclosed 

 in circular boxes, about eight inches long, which press up- 

 wards the square slide bearing the rollers. The springs are 

 moderated from below, by a screw at the lower end of the cy- 

 linder. These counterpoising arrangements are fastened to 

 the inner sides of the pillars, in the same manner that the 

 pans of the transits rising between the pillars usually are. 



There is neither semicircle, nor alhidade, nor clamp of any 

 kind, to keep the transit in a certain position ; as all arrange- 

 ments of that kind are very apt to disturb the verticality or 

 accuracy of the circle described by the transit ; which indue- 

 ed Dr. Maskeline to remove them from the transit of tlie 

 Greenwich Observatory, and to substitute in their place an 

 optical arrangement. 



In these transits the pointing in altitude is performed by 

 two semicircles, one on each side of the eye piece, on which 

 levels move by friction, around their centre, with verniers 

 and all proper adjustments. These circles are numbered, so 

 as to show altitudes, as they could not be adjusted to decli- 

 nations or polar distances, on account of the unknown lati- 

 tudes of the future observatories. The verniers being there- 

 fore placed upon the proper altitude, the telescope is turned 

 upon its axis until the level is horizontal, when the star in- 

 tended to be pointed at will appear in the telescope, — one 



