208 



MEASURING APPARATUS OF THE 



I have now given you a description of all this apparatus, which you 

 will see is very complicated. It is only the second which has as yet been 

 constructed, and has been made after the model of that used in the base 

 line near Loch Foyle, in Ireland. I wish I had the particulars of that base 

 line with me, because I am persuaded they would have been very interest- 

 ing to you, but as that is not the case, I must content myself with doing 

 the best I can by way of description from memory. 



The different parts of the base were tried against each other some- 

 what in the way shewn in the margin. 



The points A, and B, are the limits of the base in which a, b, c are 

 intermediate points : a', h', c' are points chosen on the left, a" , ¥, c" on the 

 right. The angles about A, a, h, c, B, are observed with an instrument as 

 well as those at a', a" , h', V, c', c" , so that taking any one intermediate 



distance, that from a to b for in- 

 stance, it will be determined in three 

 different ways independent of each 

 other, viz. 1st, actual measurement; 

 2nd, by the triangle a, a', b ; and 

 3rd, by the triangle a, a", b. Some of 

 the intermediate distances are tried 

 by five, some by I think as much as 

 seven computations, and the greatest difference does not exceed 25 foot, 

 or three inches in as many miles, in which are included the errors of cen- 

 tering the instrument wherewith the angles are observed. 



' The apparatus now before us was subjected to a full trial by order of 

 the Hon'ble Court of Directors in April last, under my direction, in the 

 spot called Lords' Cricket Ground in St. John's Wood road. I measured 

 nine sets of bars with the assistance of Mr. Barrow, Mr. Western, and 

 the Astronomer at Madras, Mr. Taylor, a party of the R, E. I. Volunteers 



