456 DR. C. CHREE, EXPERIMENTS ON ANEROID BAROMETERS 



In Table VIII. C denotes the values calculated in this way ; O, those actually 

 observed. 



TABLE VIII. Sums of Differences Descending less Ascending Readings. 



The differences between the observed and calculated values in Table VIII. are of 

 the same order as those met with in different experiments over the same range under 

 the most favourable conditions. 



14. The preceding results go a long way to secure one of the principal objects 

 of the investigation the more complete utilisation of the ordinary Kew test. 

 Suppose, for instance, an aneroid has been taken over any convenient range in the 

 usual way, and a summation made of the differences of the observed descending and 

 ascending readings. This sum, being based on a considerable number of independent 

 readings, is but little affected by errors of observation, if ordinary care is used. It 

 is also but little affected by any ordinary irregularities in the sub-divisions, and thus 

 affords a satisfactory basis for a general diagnosis of the quality of the instrument so 

 far as after-effect is concerned. 



By means of 11, 12, and 13 we can deduce from the observed sum, or from the 

 mean difference, the values of the corresponding quantities for any other range. In 

 this way a common standard of excellence could be utilised for all aneroids of given 

 type, irrespective of the range covered by their graduation. 



15. In the laboratory one can change the pressure at a uniform rate and keep 

 the temperature within narrow limits. In the field, however, the conditions are very 

 variable. It was thus obviously desirable to aim at a fuller knowledge of the 

 properties of aneroids, which might be of service to a wider circle than that concerned 

 with the mere testing of these instruments. 



Some valuable work in this direction was done many years ago by Dr. BALFOUR 

 STEWART, and more recently by Mr. EDWARD WHYMPER. Neither writer, however, 

 made any very serious attempt to ascertain the exact connection between the several 

 phenomena. I thus postpone consideration of their work to a later stage, when its 

 significance will be more easily understood. 



