new standards of weights and measures. 45 



hot until after several days that I discovered the cause of 

 this perplexing circumstance. I found that by placing a card, 

 the thickness of which w^as accurately one-fiftieth of an inch, 

 under the middle of the standard, the distance of the dots 

 was much increased, and by placing a card of the same 

 thickness under each of the extremities, and withdrawing 

 that which was under the centre, the distance of the dotsi 

 was considerably diminished. The total difference amounted 

 to no less than ,0016 of an inch, whilst the double of the 

 error which would have arisen from mere curvature under 

 similar circumstances would not have been one ten-thou- 

 sandth of an inch. 



The cause was now evident ; by elevating the middle of 

 the standard, the under surface was shortened, and the upper 

 surface extended ; and on the contrary, when the extremities 

 were elevated the upper surface was compressed and the 

 lower surface lengthened ; the quantity of the effect evidently 

 depending upon the thickness of the bar. 



Having thus assured myself of the source of the error, 

 a method of obviating it soon presented itself. As the upper 

 and under surfaces of the bar are in different states, the one 

 being compressed and the other extended, there must be an 

 intermediate plane which suffers neither extension nor com- 

 pression, and this plane must be nearly midway between the 

 two surfaces. I therefore caused Mr. Dollond to reduce 

 the thickness of the bar for the distance of an inch and three 

 quarters from its extremities to one half ; the gold disks and 

 plugs were then inserted as before, and the adjustment 

 completed in the manner which has been described. The 

 plugs being secured, and the projecting parts removed, the 



