518 Col. Hewry James, on the [April 23, 



of the Ordnance standard in tei*ms of the new standard of length ; 

 and for this purpose he had an intermediate standard constructed 

 on which 3-^ lengths of the standard yard were set off, and the 

 10 feet thus derived compared with the original 10 foot Ordnance 

 standard. From the comparisons made between these, we have a 

 proof of the accuracy with which the national standard of length 

 has been restored ; the difference would not, in fact, amount to 

 more than the y g^th of an inch in a mile. 



With the 10 foot standard for reference, the late General Colby, 

 who for many years so ably superintended the survey, designed his 

 Compensation Bars, for the measurement of the base lines for the 

 triangulation. General Colby, availing himself of the known 

 unequal expansion of brass and iron, combined bars of these metals 

 in the base-measuring apparatus, in such a way as to preserve 

 the distance between two points on the tongues connecting the 

 bars, at the constant distance of 10 feet, under every change of 

 temperature. 



In the actual measurement of the bases, these bars were ranged 

 in a perfectly straight and horizontal line, and to prevent any 

 possible disturbance in the position of the first laid bars, they were 

 separated by an interval of six inches, the interval itself being 

 measured with a double microscope, the foci of the microscopes 

 being exactly at six inches apart, and their invariability secured by 

 the bars connecting the two, being made to compensate each other's 

 expansion, in the same way that the 10 feet bars are compensated. A 

 central microscope between the two described serves as a pivot for 

 reversing them, and also for the purpose of establishing fixed points 

 on the ground, as points of reference in the remeasurements taken. 



Sir John Herschel and Mr. Babbage were present when 500 

 feet of the base at Lough Foyle were remeasured, and the error 

 amounted to only a third (by estimation) of the breadth of the very 

 finest dot which could be made with the point of a needle. 

 We have thus an assurance of the extreme accuracy with which 

 the two bases, one on Salisbury Plain, the other on the shores of 

 Lough Foyle, in the north of Ireland, each about seven miles long, 

 were measured. These base lines may, in fact, be described as air- 

 lines drawn from the fine dot at one extremity to that at the other. 



Having established an accurate base, the next operation is to 

 establish some trigonometrical stations to form triangles with it ; and 

 then, by means of a theodolite, the centre of which is accurately 

 adjusted over the dots at the extremities of the base, measuring the 

 angles between the stations, the data are obtained for computing the 

 length of the sides of the triangles. The sides of the triangles thus 

 obtained become new bases, from which the length of the sides of 

 other triangles are in like manner computed ; and in this way the 

 exact length of every line in the great network of the triangulation 

 is accurately known. Of the accuracy with which the angles were 

 taken, we have, first, the proof by the summation of the angles in 



