of Van Diemen's Land. 85 



from the adoption of the decimal scale. The reductions to 

 the horizontal and to the level of the sea were made as 

 hefore. A third measurement was undertaken immediately 

 on the completion of this with the same means and the 

 same careful attention. 



The three several measurements gave the followiug 

 results : — 



Measurement of 1849 20182-484496 feet. 



First measurement of 1851 20181-692922 „ 



Second measurement of 1851. ...20181-577215 „ 



The difference between the measurement in 1849 and 

 the mean of those in 1851 is "85 foot, or ]0| inches; 

 that between the two in 1851 only -116 feet, or 1^ inch. 

 I consider this latter accordance to prove that the slight 

 variations in the length of the rods were truly valued, and 

 the apparatus in all respects sufficiently true, and that 

 Mr. Sprent, who conducted the whole operation, has ob- 

 tained the nearest results obtainable by its means. 



It also seems satisfactorily to show that the first mea- 

 surement may be rejected, though (considering the com- 

 parative inferiority of means) not very greatly differing 

 from what will be adopted as the actual length of the Base, 

 viz., the mean of the two last measurements. 



The Base of verification on Norfolk Plains has since 

 been measured twice by the same apparatus, and to the 

 same standard, by Mr. Sprent ; the results of which are as 

 follows : — 



First measurement, reduced to the 



level of the lowest point 25746-019443 



Second ditto ditto 25746-304833 



Difference -285,390 = 31 in. 



The difference of 2>\ inches in nearly five miles is almost 



