292 Notices respecting Nexv Booh. 



manner. The differences between the computed and measured 

 lengths were found to be exceedingly minute, excepting in one case, 

 where the proportionate error amounted to a foot in 9'8 miles. It 

 is not likely, however, that the probable error of the whole length 

 exceeds 3 or 4 inches. 



Unusual precautions were taken for the purpose of preserving the 

 base points, so that they may be available, if required, at any 

 future time. And this was done not only at the two points where 

 the measurement commenced and terminated, but also at an inter- 

 mediate station (Minearney), and at Mount Sandy, to which the base 

 was prolonged by triangulation. At each of those four points a 

 circular piece of ground, to the extent of 30 feet in diameter, was 

 purchased by the Ordnance and enclosed. The foundations of the 

 stations were laid in solid masonry. " When the exact position of 

 the points had been determined, jumper-holes, l^ inch in diameter 

 and 6 inches deep, were bored in the upper blocks, and pieces of 

 platina wire, ■—- of an inch in diameter, were inserted in the jumper- 

 holes, and retained in a vertical position by fine wire twisted round 

 each, with ends projecting to the sides. The holes were then run 

 in with lead to within about one inch of the surface of the stone. 

 This last inch was filled up with a mixture of cement and sand that 

 the jumper-holes might resemble the rest of the stones, and the 

 platina wires were cut oft" level with the surface. The dots on the 



wires were made with the point of a needle The points 



at these four stations are now being further secured from injury by 

 surrounding them with small dwarf walls, and covering each of them 

 with a tumulus of earth ; another dwarf wall, with an iron railing, 

 will encircle the space at each station purchased by the Board of 

 Ordnance." — P. 152. 



The above precautions were no doubt justified by the importance 

 of the object, for it would seem that this liOugh Foyle base, upon 

 which the triangulation of Ireland depends, is to be assumed also as the 

 unit of the measure of the whole British arc of meridian from Dunnose 

 to Balta. Considering the great precision now attained in such ope- 

 rations, we are not prepared to say that a verificatory base is indis- 

 pensable, even for an arc of 10°; yet, we should be sorry to see it 

 dispensed with in the present case. The bases measured by Roy 

 and Mudge in the early stages of the survey will not, we suppose, be 

 considered as sufticiently accurate for this purpose, and ^'^ so, it 

 would seem to be very desirable that another base should still be 

 measured somewhere in the south of England, and nearer to the line 

 of the meridian. 



All the details connected with the measurement, both with the 

 bars and by triangulation, are very fully given ; and we have also 

 the results of a great number of experiments, made in diff^erent 

 years since 1827, for ascertaining the relation of the different sets 

 of compensation bars to the standard 10-foot iron bar; of this bar 

 to another similar bar, and to various standards of length which 

 have acquired authority or reputation from the comparisons to 

 w'hich they have been sulijcctcd ; of the microscopic aj)paratus to 



