MEASURING APPARATUS OF THE 



The bars used by the French Geodists in measuring their bases are 

 of a peculiar construction. They are of platina, two toises in length, 

 6 lines in breadth and about 1 line in thickness. Each of them is sheathed 

 with a bar of copper within 6 inches of the extremity. The two plates of 

 platina and copper are united at one end whilst the other is at liberty, 

 and as the expansions of these two metals are nearly in the ratio of 1 to 2, 

 it is evident that the unconnected end of the copper bar will advance or 

 recede from that cause on the platina ; the quantity being the difference 

 of the increments or decrements due to the whole length of the bar. This 

 then would be a thermometer of a most exquisite and delicate structure, 

 provided the two metals radiated equally, and that the heat were equally 

 diffused through the mass of each, but it is evident that such could never 

 be the case unless the apparatus be sheltered from the direct action 

 of the rays of the sun, and all other causes of inequality of tempera- 

 ture. 



We come now to the measurements by General Mudge with Ramsden's 

 steel chain. In 1784 trials were made on Hounslow Heath, of a new chain, 

 a set of deal rods, and a set of glass rods, on the •21st June of that year, a 

 distance of 7.800 feet was measured and re-measured with the chains, and 

 the return measurement differed from the 1st by 1^ inch, that is, I inch 

 per mile. On the morning of the 18th August, a space of 1000 feet was 

 measured with the steel chain, and with a set of glass rods, and the differ- 

 ence after reduction for temperature was only -02 of an inch, a quantity 

 incredibly small. The remainder of the Hounslow Heath base was mea- 

 sured with glass rods. 



On Monday, .30tli August, the measurement with the glass rods was 

 completed, being after all reductions made, 27404,0137 feet at the level of 

 the sea ; so that the daily rate of progress was 2,108 feet. Here the soil 

 is so level that the rods lay on the ground without support. 



