24 PHILOSOPHICAL TRANSACTIONS. * [aNNO 1785. 



two most fiimous observatories in Europe, Greenwicli and Paris, would be more 

 accurately ascertained than they are at present. The execution of this business 

 was confided to the care and diligence of Gen. R., which naturally divides into 

 2 parts. First, the choice and measurement of the base, with every possible 

 care and attention, as the foundation of the work; 2dly, the disposition of the 

 triangles, by which the base is to be connected with such parts of the coast of 

 this island as are nearest to the coast of France, and the determination of their 

 angles, by means of the best instrument that can be obtained for the purpose, 

 from which the result or conclusion will be drawn. 



With regard to the first of these, the choice of the base, it is observed, that 

 Hounslow-heath having always appeared to be one of the most eligible situations 

 for any general purpose of the sort now under consideration, because of its vici- 

 nity to the capital and Royal Observatory at Greenwich, its great extent, and 

 the extraordinary levelness of its surface, without any local obstructions whatever 

 to render the measurement difficult; being likewise commodiously situated for 

 any future operations of a similar nature; accordingly a particular inspection of 

 the heath was made, to assign and trace out a proper position for the purpose. 

 Gen. R. then minutely describes the clearing of the ground, and the construc- 

 tion of the steel chain and other instruments employed in the measurement of 

 the base, which must be performed with the utmost care. 



After the description of the chain, which consisted of 100 links of 1 foot 

 each, the measuring rods are next described. The bases which had hitherto been 

 measured in different countries, with the greatest appearance of care and exact- 

 ness, had all, or for the most part, been done with deal rods of one kind or 

 other, whose lengths being originally ascertained by means of some metal standard, 

 were, in the subsequent applications of them, corrected by the same standard. 

 Having thus had so many precedents, serving as examples to guide them in their 

 choice, it was natural enough that they should pursue the same method in the 

 measurement to be executed on Hounslow-heath; taking however all imaginable 

 care, that the rods should be made of the very best materials that could be pro- 

 cured; with this further precaution, that by trussing them, they should be ren- 

 dered perfectly inflexible, a circumstance not before attended to. Three mea- 

 suring rods were accordingly ordered to be made, and also a standard rod, with 

 which the former were from time to time to be compared. Their stems were 

 each 20 feet 3 inches in length, reckoning from the extremities of the bell-metal 

 tippings; very near 2 inches deep; and about ]-l inch broad. Being trussed 

 laterally and vertically, they were thus rendered perfectly, or at least as to sense, 

 inflexible. 



Next follows a description of the brass standard scales employed in setting oflf 

 the length of the deal rods. At the sale of the instruments of the late inge- 



