OF ARTS AND SCIENCES. 189 



they were ruled and filled, but between the 1st and the 10th of 

 April of the present year nearly one half of the powder was com- 

 pletely removed from the lines. 



It is possible also, that some of the discordances are due to the fact 

 that the measures were not all made along the same horizontal line. 

 My earlier measures were made along a line just above the transverse 

 line, while the later ones were made along a line a little below the 

 transverse line. My observations of April and May of the present 

 year, and those made by Mr. Edmands, ought to be comparable, since 

 they were made at nearly the same time and along the same line. 

 The greatest disagreement is in the third space ; and that the dis- 

 cordance is not an accidental one is shown by the fact that all my 

 observations agree in giving — 27, while all of his agree in giving 

 — 16. But, admitting that the discordances are all due to errors of 

 observation, it will be seen that the average deviation from the mean 

 is only 6 millionths of a centimeter. 



