o 



•iS DENSITIES OF OXYGEN AND HYDROGEN 



Cleveland. Since the bars, taken with the correction certified by the Socicte 

 Gcnevoise, give the same value for the metre within the limits of the precision of 

 the ol)servations, the length of my bar is [lerhajJS well eiiongh known. At l*(>.'.>", 

 by observations with two theiniometers on each Ijar, in a room whose tenijteratur.e 

 varied half a desri'ee duriiiij the si.x hours of the observations, mv bar was found to 

 be l.UOUlH metres in length. If we assume for my bar the coefficient of expan- 

 sion of glass used in computing the table for reducing the height of the barometer 

 given in tlie compilation of Landolt and Boernstein, and with this compute the 

 length at <J°, the value obtained Avill be uncertain to some extent, but it will repro- 

 duce the true value at 20.9", and w ill enable us to use these tables for the necessary 

 reductions. We then have 



Length of my bar at 20.9° i.oooi 14 



Expansion for 20.9", = 20.9 X .0000085 000178 



Length of bar at o ' 999936 



Tiic errors of the graduation were found to be insignificant foi' oui' piiipose. 



The observations were therefore reduced by multiplying the observed pressure by 



the factor 



.999936* 



17. THE UALANCE USKI) IX I'HK FIRST SERIES OF K-XPERUIENTS. 



The balance used in the present series of experiments was made by Becker of 

 Rotterdam, and will cai'iy 1200 grammes in each ]>an. It was procured for this 

 investigation, and has been used for nothinur else. Duiino; these experiments, it 

 was mounted on a case of nonconducting materials, thirteen centimetres thick, with 

 doom of the same thickness. It was placed in a small room containing no soinve 

 of heat, but surrounded on all sides with loonis kept as nearly at constant tempera- 

 ture as is usual in large buildings. 



Much difficulty was experienced at first in determining the difference of weight 

 Vjetweeu the globe and its counterpoise with sufficient accuracy and witliin a 

 reasonable time. This difficulty is mostly due to currents of air affecting une(pially 

 the two globes, or, possibly, the two pans of the balance. Tlie li.il.inee itself was 

 enclosed in a case which hail doors for manipulating the weiglils, and had also 

 two openings in front, each some three centimetres in diameter. The lower of 

 the.se contained a lens by which light could be conden.sed on the scale over which 



* .Since this paper was written, Professor Dayton C. Miller has had these two metres com- 

 pared by the United States Coast and Geodetic Survey with the national prototype metre, which is 

 in its custody ; with the result that they are a thirty-thousandth i)art too short. It is too late to 

 correct the reduc'.ion of each observation on density which is mentioned in this paper, but the 

 means of each series, and the final values, have been corrected by increasing each by a thirty- 

 thousandth j)art, 



