220 MESSRS. U. THUELFALL AND J. A. POLLOCK 



reading at a third station and the corresponding temperature readings at each station, 

 give us all the data necessary for calculating the value of gravity at the third station. 

 The necessity of observing the temperature arises from the fact that the rigidity 

 of fused quartz threads increases with increase of temperature. The effect is, of 

 course, complicated by the changes which take place in the dimensions of the threads. 

 The nett result is that as the temperature rises the " stiffness " of the threads 



increases.* 



The dimensions of the lever also change, and in such a manner as to partially 

 compensate the changes of "stiffness" taking place in the thread. In the actual 

 instrument the resilience as well as the figure of the coach spring vary with 

 temperature, the line of collimation of the microscope may also vary relatively to the 

 level, and the metal framework may also twist and vary in shape in any manner. 

 All these effects are found to be capable of collection into a single temperature 

 coefficient which, so far, we have found to be sufficiently well expressed by a single 

 term the temperature scale adopted being the platinum scale of CALLENDER. 



Many attempts were made to annul the effect of variation of temperature by con- 

 structing levers composed of two bars of metals of different expansibilities, an account 

 of which will be found in the appendix, but all such attempts came to nothing. 



The sensitiveness of the instrument is at present such that to compensate for a 

 change of one part in 100,000 in the value of g it would be necessary to move the 

 vernier arm through 2'12 sextant (doubled) minutes = TOGO minutes of arc. Also a 

 change of temperature of one-tenth of a degree alters the circle reading by 3 '15 

 sextant minutes, gravity being invariable. 



Although quartz is immeasurably superior to any other known material as regards 

 the constancy of its elastic properties, it must not be supposed that it is entirely free 

 from elastic defects. As a matter of fact, it is only just good enough for the present 

 purpose. One of the great difficulties we have had to overcome has arisen from the 

 fact that it is only by the most judicious choice of dimensions that it is possible to 

 reduce the viscous yielding of the thread to within practicable limits, and even as it 

 is, although the thread of the present instrument has been twisted for more than two 

 years, and indeed purposely overtwisted for part of that time, we are still obliged to 

 apply an important though practically constant correction on this account. In other 

 words, the reading of the sextant arc is still slowly decreasing, and though the rate 

 of decrease is now constant for all practical purposes, it has to be taken strictly into 

 account in interpreting the indications of the instrument, even when consecutive 

 observations are separated by an interval of time as short as a single day. 



It will, no doubt, occur to the reader that we ought to use a finer thread, in order 

 to get rid of this source of inconvenience. This, however, experience has shown us, 

 we are not at liberty to do, for a finer thread means either a finer lever or one 

 soldered to the thread at a point nearer to its own centre of gravity. The former 



* THRF.T.FAU , 'Phil. Mag.,' July, 1890. 



