352 Ordnance Survey Astronomical Observations. 



dredth only of a second of a degree of latitude. But the astro- 

 nomical determination, on the other hand, even with the best 

 existing instruments and methods yet invented by man, can 

 hardly be brought within several tenths of a second ; so 

 great are the practical difficulties which beset the subject. 



All possible means and appliances should therefore be em- 

 ployed, and even exhausted, in improving the accuracy of the 

 astronomical determinations of the latitudes of the terminal 

 points; and these when once well known, allow of the latitudes 

 of any and all the other points in the country being computed 

 from the terrestrial measurement with equal accuracy, and 

 of being duly inserted in the margin of the maps. Not 

 only, therefore, our knowledge of the compression of the 

 earth depends on these terminal points, but also the latitudes 

 of every place in the country, and the indications of every 

 single sheet of the thousands of which the survey consists. 

 The lines of latitude and longitude extending over a coun- 

 try are in fact an astronomical network, all the lines of which 

 are obtained by computation from, and hinge on, the two or 

 three stations, at which alone it has been possible to get the 

 long and troublesome series of astronomical observations 

 well performed. With any errors in the so determined posi- 

 tions of those spots, every other in the whole length and 

 breadth of the land are out also ; and a greater uncertainty 

 still is introduced into the results for the size and shape of 

 the earth ; which again reacts prejudicially on the correctness 

 of the formula employed in the calculations of all the triangles 

 of the terrestrial portion of the survey. Hence the devotion 

 of 1000 pages of closely-printed figures to the observations 

 for determining the astronomical latitudes of a very limited 

 number of places in the country. 



Were perfect observation possible, two stations would have 

 been enough in Great Britian, one in the extreme south, and 

 the other in the extreme north. But as such a comfortable 

 state of things can exist nowhere under the sun, the whole 

 meridian length has been divided into several parts, and there 

 are meridian lengths also taken on either side of the king- 

 dom, so as to have as many separate results as possible ; 



