Ordnance Survey Astronomical Observations. 353 



and moreover, at each intended station, observations have 

 been made at several spots round about it, so as practically 

 to ascertain whether there be any local attraction in the 

 neighbourhood that might prejudice the results. 



In this way the number of stations observed, has been 

 increased to 26, extending from Saint Agnes in the Scilly 

 Islands, in 49° 53', to Saxavord in North Shetland, in 60° 49' 

 N. lat. 



The first step in this inquiry was, of course, to get a good 

 astronomical instrument, and the zenith sector made by 

 Ramsden for the Ordnance having been destroyed in the fire 

 at the Tower in 1840, application was made to the Astrono- 

 mer Royal, G. B. Airy, Esq., to furnish a design for a new 

 one, with any improvements he could suggest. 



The result of this was the instrument employed, wherein the 

 following capital improvements were introduced over all other 

 previous zenith sectors — 1st, the whole being made of metal, 

 and in large pieces with few adjustments ; 2d, A double 

 graduated arc, or readings, and with micrometer microscopes, 

 at either end of the telescope, which turned on an axis in the 

 middle of its length ; 3c?, Levels were employed for the zero 

 point in place of the old plumblines ; and 4cth, The whole ar- 

 rangement was such as to admit of the double observations, 

 face E. and face W., being made at the same transit. 



The length of the telescope was 46 inches, the diameter of 

 the object-glass 375 inches ; the magnifying power usually 

 employed 70 ; and the weight of the whole instrument, 10 cwt. 



This was screened from the weather by a portable wooden 

 building, covered with canvass, and having an observing slit 

 in the roof, and weighing 23 cwt. 



From 1842 to 1844 the observations were made by com- 

 missioned officers, Captains Hornby, and DriscollGosset,R.E., 

 but subsequently, when the modus operandi was well settled, 

 they were made and computed by non-commissioned officers, 

 under charge of Captain Yolland, R.E. 



On the average, 800 observations of stars were taken at 

 each station, and each observation is given in the body of the 

 work, nearly as read off from the instrument itself, with all 

 the necessary elements for its reduction, and the principal 



