'374 PROF. C. p. SMYTH ON THE REPUTED METROLOGICAL SYSTEM 



at the Pyramid, but comparisons of the scales or rods employed there with those 

 used in modern geodetic operations. These unfortunately we have not directly, 

 and in all the cases which I have had an opportunity of examining, there had 

 been a shortening of the scale in a hot, desert country. On a wooden scale, 

 the dry weather might account for this ; but on a steel rod where it certainly 

 existed, it seemed only explainable by a slow contraction of the metal, gradually 

 recovering itself from the operations of extension which it had undergone by 

 hammering and rolling at the forge. 



The French savans ought to have guarded against these sources of error, and 

 their observation is evidently by far the more carefully taken of the two pre- 

 viously cited ; and gives, reduced to inches, for the -^l^th part of the double base- 

 breadth, multiplied by 10 millions, 



500,733,000 ; 

 Or, in using the a^^i^^^, 500,740,000. 



Howard Vyse's measure, similarly treated, gives 



500,984,000, 

 and 500,990,000. 



But correcting his measure, by the proportions which his numbers for " the coffer" 

 bear to those of Prof. Greaves, and correcting Prof. Greaves' measures by the 

 French determination of the length of the foot which he engraved in the Pyramid 

 on granite, immediately after measuring the coffer, and which foot is stated by 

 M. Jomard, in the " Description de I'Egypte," to be 0-30460 of a metre (of 

 which a whole length is equal to 39*37079 English inches), the above quantity 

 should be reduced to 



499,915,000, 

 and 499,921,000 ; 



and the mean becomes, giving twice the weight to the French measure, as seems 

 at least its due, from the superior care taken with it, 



500,528,000, 



and 500,535,000 ; 



^ ' Mean = 500,532,000. 



Now this quantity is quite contained within the variations of modern observa- 

 tions of the earth from each other, even in the best and latest geodetic com- 

 parisons; for they are given in the shape of different determinations of the 

 length of the earth's polar axis in inches, by De Schubert, as varying between 



500,560,000, 

 and 500,378,000 ; 



but with much more inclination toward the former than the latter ; and he is not 

 materially differed from by the results of Bessel, Airy, Herschel, and Pratt ; the 

 latter, by his last refinements of taking into account the local disturbances at eachj 

 astronomical station, making the polar axis equal to 500,523,000 inches nearly ; 

 while the others are all rather under 500,500,000. 



