On Gravimeters. 



517 



enlargements above and below. The lower end is closed by a flexible 

 corrugated steel plate, which has a solid central disk, by which the 

 pressure on the plate is transferred to the cross-bar. When this 

 plate is horizontal, the mercury presses equally on every part ; but it 

 is clear that the whole of the pressure is not borne by the cross-bar. 

 In fact, it would only by trial be possible to ascertain what weight is 

 borne by the springs. Any change in the position of the disk must 

 alter the curvature of the plate, and with it the proportion of the 

 weight borne by the springs and by the rim of the cup — irrespective 

 of any change in the height of the column. The diameter of the 

 steel tube would seem to be adjusted so as to cause a proper change in 

 the level of the upper surface, when the bulk of the mercury is 

 affected by change of temperature. I am not quite sure that, in 

 presence of these several causes of variation, the principle of con- 

 stancy of weight on the springs can be said to be retained. 



Alterations of position of the disk are read by means of a micro- 

 meter below the cross-bar. It seems clear that the desired indications 

 must be obtained from the readings of this contact micrometer, and 

 some curiosity may be experienced as to • the manner of interpreting 

 its readings. On this point the author says : — " It would be difficult 

 to determine the actual scale of the instrument d priori ; and I there- 

 fore adopted the easier and safer method of relying for its final 

 adjustment upon the result of actual working." 



If it were possible to ascertain, with some approach to certainty, 

 the weight borne by the springs in some particular position of the 

 diaphragm and cross-bar (to be called the zero position) the instru- 

 ment might be used, on the principle of the simple gravimeter, by 

 ascertaining what weight upon the cross-bar would be necessary, under 

 a diminished gravity, to bring it again to this zero position — tempera- 

 ture being the same. This, however, is not what is intended in this 

 form of instrument. 



The subject of compensation for temperature receives considerable 

 attention. This, indeed, is very necessary, when we remember the 

 sensitiveness of springs to heat. The author gives, as the result of 

 experiment upon these springs, a factor of variation which appears to 

 be 40 00 for each degree Centigrade. The importance of temperature 

 in such case will be better understood if we consider that at this rate 

 it would need but 21° C. to effect as great a change in the elastic 

 force, as transference from equator to pole would effect in gravity. 

 Change of temperature has here, it would seem, an effect twelve 

 times as great as it has in the case of the pendulum. Certainly there 

 is room for compensation ; and Dr. Siemens obtains it, to some extent, 

 by apportioning the diameter of the vertical tube to the cups. It is 

 not necessary, for the present purpose, to consider closely how this 

 exercises a compensating effect. It is estimated, by experiment, that 



