AT KEW OBSERVATORY AND THEIR DISCUSSION. 491 



Putting I = '83, and m = T3, as in Table XLIL, we find 



k = '684 -r- (54-6 + 91-9 37'8) = '0063. 



This is a mean value for the 13 aneroids dealt with as a group in Table I. ; 

 individuals of the group differed, of course, amongst themselves. 



46. In the case of the experimental aneroids, it might appear that m and I are 

 at once determined by the experiments. 



For instance, it might be supposed that 



fall of reading in 10 minutes at lowest point 

 tn * > 



fall of reading in 5 minutes at lowest point 



recovery in first 5 minutes at end of cycle 

 fall in first 5 minutes at lowest pressure 



If this were so, we should have from the first 24 experiments for the mean of 

 aneroids Nos. 1, 2, 3, and 4, 



m = 73/47= 1-55, 

 I = '65, roughly. 



In the case of tn, if we preferred to use the formula (8), we should have 



m = 73/57 = 1-28. 



These deductions, however, assume the creep phenomena the same whether 

 pressure is steady or changing, which may not be strictly true. Further, even if 

 there were no theoretical objection, the method would not, in practice, be very 

 satisfactory, owing to the large probable error involved in determining such small 

 quantities as an increment or decrement of creep in 5 minutes. 



Another method that naturally suggests itself is a comparison of the observed 

 sums of the differences of the descending and ascending readings in three ranges. 

 This supplies two simple equations involving I and m as unknowns. This method, 

 however, is in practice no better than the other, as will be recognised on inspection 

 of Table XLIII. The quantities tabulated there are the ratios borne by the sums of 

 the differences of the descending and ascending readings over several ranges to the 

 sum for the range 30-21 inches. The observed values are taken from the first 

 24 experiments. 



3 K 2 



