254 C. S. Peirce — Oscillation of Pendulums. 



sulphates, dissolving in cold water, adding hyposulphate of 

 soda, and boiling; the precipitated thoria was collected on a 

 filter, washed, ignited and weighed (Hermann's method.)* 

 The amount of cerium was arrived at by determining the 

 excess of oxygen in CeO a over Ce a 3 : this was obtained by 

 dissolving the ignited oxides (free from thoria). in dilute sul- 

 phuric acid mixed with oxalic arid ; the oxides dissolved and 

 the higher oxide of cerium decomposed the oxalic acid, setting 

 carbonic anhydride free, which was collected in potash bulbs 

 and weighed. Silica was determined by decomposing the min- 

 eral with sulphuric acid, evaporating till fumes came off, soak- 

 ing out with water and filtering. 



A determination i ide in a weighed quan- 



tity of the mixed sulphates of the cerium metals from the 

 Portland variety gave for the joint molecular weight of the 

 oxides 328-2, or a joint atomic weight of 140-1, which deter- 

 mination was used in the calculation of the three analyses. It 

 may be stated that great care was taken to prove the identity 

 of the thorium. Besides its giving all the reactions for that 

 element, a weighed quantity of the oxide gave by conversion 

 into sulphate an atomic weight of 238'5. This sulphate was 

 not quite soluble in water, which readily accounts for the 

 result hciim a trifle higher than that usually accepted for thoria. 

 The atomic weight, 231 -5, was used in calculating the analyses. 



In closing I wish to express my thanks to Professors George 

 J. Brush and E. S. Dana for kindly providing the material for 

 carrying out this investigation. 



of Pendulums; by C. S. Peirce. 



Geodetic Survey.] 

 The pendulum experiments conducted by me for the Coast 

 Survey exhibit considerable dilforences in the rate of descent 

 of the arc on different days. Mr. O. T. Sherman (this Journal, 

 xxiv, 176) having Mig^esied that this might be due to a 

 periodic variation of the amplitude, I feci called upon to say, 



this supposition is inadmissible. For, in order to account, in 

 this manner, for the observed discrepancies, it would be neces- 

 sary to suppose a periodic variation too great to escape direct 

 observation. In most of my observations, I have used an arc 

 accurately divided into thousandths of the radius. The read- 

 ing telescope has a sidereal magnifying power of from 60 to 



* Jouro. f. pr. Chem., xxxiii, 90. 



