420 



Mr. C. F. Brush on the Measurement 



determinations : — " Following is the last reading in detail, 



snowing 



the extrem 



e accuracy oi 



tries 



e measuremen 





•432 M. 



•438 



M. 



•441 M 





•441 „ 



•4335 



j? 



•429 „ 





•4335 „ 



•4275 



jj 



•4305 „ 





•426 „ 



•450 



?? 



•435 „ 





•4335 „ 



•4425 



>j 



•432 „ 





•4395 „ 



•432 



?? 



•4185 „ 





•4305 „ 



•435 



>; 



•435 „ 





•441 „ 



•432 



jj 



•453 „ 





•435 „ 



•4215 



7? 



•4425 „ 





•435 „ 



•4245 





•438 „ 



Means... 



•4347 „ 



•43365 



•43545 „ 



" Mean of all the readings, -4346 M. 



" Eeadjusted zero-point of micrometer before each reading 

 of each set. Partially emptied gauge and readjusted capillary 

 depression before each set of readings. The first series has 

 no known source of error. The second and third series were 

 made during wind-squall, and surface of mercury was often 

 tremulous. In the third series, capillary depression was per- 

 ceptibly, though very slightly, unequal, in direction to make 

 readings too high." 



In the above quotation, " M " means millionths of atmo- 

 spheric pressure. The calculated probable error of the thirty 

 readings taken together is only nineiy-two hundredths of a 

 unit in the third decimal place ; that is to say, less than a 

 thousandth part of a millionth of atmospheric pressure! The 

 probable error of the three mean results, considered as single 

 readings, is only eleven hundredths of a unit in the third 

 decimal place of millionths. The net result may be expressed 

 as follows, in terms of atmospheric pressure: — Considered as 

 thirty measurements : 



0000 000 434 60 + 0-000 000 000 92. 



Considered as three measurements : 



0-000 000 434 60 + 0-000 000 000 11. 



Here we have the measurement of a total quantity of less 

 than half a millionth of atmospheric pressure, with a probable 



