to ineafure an Arc of the Meridian. 2 7 s 



fummit of this angle, poles in the direction of each of the 

 fignals were erected, at the diftance of every hundred toifes. 

 At the bottom of each of thefe poles, a wooden wedge was 

 driven into the ground to fuch a depth that nothing could 

 alter its pofition, and the place of each of the wedges was 

 marked by heaps of (tones difpofed along the road. 



After thefe preliminary labours, Delambre began the ope- 

 ration of meafurement on the 5 th of Floreal, at the bafe of 

 the fignal of Melun. The inftruments conftrufted for this 

 piirpofe were four rules of platina, each two fathoms in 

 length (at a given temperature) ; fix lines in breadth, and 

 two thirds of a line in thicknefs. Each rule was mounted 

 on a piece of wood, fufficiently ftrong to be fecure againft 

 all warping and bending ; and was covered, at the diftance 

 of three inches, with a flight cafe of wood, at the two 

 extremities of which arofe two points made of iron, to ferve 

 as marks to direft the fight. On the rule of platina there 

 was placed another rule of copper of the fame ftrength, but 

 fomewhat lefs in length, which ferved to meafure the ex- 

 panfion of the platina. It is well known by experience, that 

 the expanfion of platina is to that of copper as 12 is to 25. 

 The two rules were fixed upon each other, in an invariable* 

 manner by their anterior extremity, in order that the effect 

 of the expanfion might be carried entirely to the other ex- 

 tremity. The rules being plunged into melting ice, and 

 confequently at o y of Reaumur's thermometer (3 2° Fahr.), 

 the extremity of the copper coincided on the platina at a line 

 marked ; but as foon as they were expofed to another tem- 

 parature, the copper extended on the platina, on account of 

 the different expanfion of the two metals. An ingenious 

 apparatus conftruaed on this principle, afforded, with the 

 aff.ftance of a nonius and microfeope fitted to it, the means 

 of eftimating the dilatation of the platina to nearly j^oss 

 of a toife. This apparatus formed at the fame time a very 

 fenfible metallic thermometer, each part or degree of which 

 correfponded to a dilatation of 0,000009245 of a toife ' on 



eacn 



