AT KEW OBSERVATORY AND THEIR DISCUSSION. 



447 



(ii.) Making the number of points of observation the same in all ranges by 

 suitably varying the size of the pressure steps. 



(iii.) Allowing only half-weight to the two terminal differences ; for instance 

 in the range 30-21 inches taking c? as the mean of nine quantities, of which 

 (ci + c?io)/2 is one. 



The last of these changes was, of course, within my power, but as its advantages 

 after all could not be large, and might be nil, it did not seem worth while to under- 

 take the large amount of arithmetic that would have been entailed by its substitution 

 for the method originally adopted. 



Fig. 2. 



/ -i -3 -4 -5 -6 7 S -9 



Fractions of pressure r&rtge from lowest point 



10 



6. In actual practice, the change (ii.) specified above would be inconvenient, 

 entailing readings at fractions of inches difficult to determine exactly on an ordinary 

 gauge. In ranges where the points of observation are sufficiently numerous the 

 same object can be obtained pretty satisfactorily by interpolation. For instance, in 

 the range 30-21 inches, the middle of the range is 25'5 inches, and (d 5 + d 6 )/2 may 

 be regarded as a tolerably good first approximation to the corresponding difference. 



