﻿522 ON EXTEITDING A GEOGItAPfllCAL SOUVEY 



It appears from the best information I have re- 

 specting it, that it was measured off by the bras* 

 standard when the thermometer stood at 6"2°, and 

 was, in that temperature, exactly 100 feet in length. 



From the want of a proper standard scale and 

 beam compasses, I \\ould not undertake to determine 

 its length, compared with brass ; because I did not 

 think that laying off any determined number of feet 

 from the sliders in the register heads, and by a pair 

 of common compasses, could be done with suf&cient 

 accuracy, so as to enable me to find out at xvhat de- 

 gree of temperature the chain had measured 100 feet 

 by the brass scale. And as I had been informed by 

 Doctor Dikwiddie, from whom it was purchased, 

 that, to the best of his recollection, it had been ad- 

 justed to 100 feet at the standard temperature of 6'i»; 

 I therefore rested satisfied until further information 

 may be obtained respecting it ; and it is probable, 

 that any correction on account of temperature, will 

 not amount to more than two or three feet, and an 

 error of that magnitude in a length of near 7s miles 

 cannot be of verv o-reat moment in geographv, Avhich 

 is the principal olject at present. 



There is another circumstance it may be neces- 

 sary to mention with respect to the chain. From 

 the same want of a standard measure, I have not at- 

 tempted to determine its wear ; but I observe that 

 in tiic measurement of the base of verification on 

 SaMshufy plains, the chain used there was very little 

 affected by being in use about seven weeks. And in 

 order to j)revent the M^ear as much as possible, I al- 

 lotted twenty coolies, that is one to every two links, 

 Mhose sole business it was to lift out the chain and 

 lay it on the ground whilst the coffers Avere moved 

 forward, and then to re])lace it when they were ready. 

 All this was done M'ith the greatest care, and always 

 by the word given them, that the motion might be 

 as trifling as possible. This mode was practised 

 wuring the whole measurement, so that I am in 



2 hopes 



