220 Finul llcmarh by Mr. Ediuard Riddle onfijiding 



whatever, except that of turning over one leaf in Mendoza Rios's 

 tables. My object was to show that in finding the tnie altitude 

 of the moon's centre above the sensible horizon, Mr. M.'s me- 

 thod was not wrong only because it was not really different from 

 that conimonly practised ; and that his method of finding the 

 apparent altitude above the same place was wrong exactly by 

 what it diffeied from the method which he was desirous of su- 

 perseding. In both these respects 1 succeeded; and setting ex- 

 perience aside, it would scarcely huve been believed that he would 

 have asserted that his crude misconceptions on this subject were 

 " sanctioned by the authoritv of Maskelyne, Pond and Brinkley." 

 On the subject of the quadrant, I pointed out the principle on 

 which Mr. ^I. proposed that the operation of the instruments 

 should be explained ; and I showed that there was nothing vague, 

 unsatisfactory, or incorrect in the explanation which is commonlv 

 given of it; and this was all I proposed to effect. Mr. M. is 

 right in observing, that " we can never be too simple in our ex- 

 planations;" but though his demonstration of the principle of 

 the quadrant is true, it has i)o very evident pretensions to sim- 

 plicity. 



We have seen that he does not correctly understand what 

 astronomers mean by the term parallax; it appears also that he 

 does not know what is meant by the augmentation of the semi- 

 diameter. Indeed in his letter of November '2d, his misconcep- 

 tions on this subject have led him to make statements too absurd 

 to he defended ; and he has for once, at least, acknowledged that 

 he w;is in an error. But the merit of this concession is mate- 

 j-iallv lessened by an accompanying circumstance. He does not 

 state the true cause of his falling into the mistake. He affirmed 

 that the augmentation was not the same for every semidiameter, 

 and he deduced this curious cojiclusion very fairly, on the sup- 

 position that the moon is fiat', this was the true cause of his 

 mistake, not that which he thinks proper to assign. 



Mr. Meikle is exceedingly out of temper at the diagram on the 

 subject of parallax, and his anger is easily accounted for. That 

 diagram altogether overturns his assertion, that the difference of 

 the parallax of any two diametrically opposite limbs is the aug- 

 mentation of the diameter ; and he cannot good-humouredly 

 look on and see such havoc made among the creations of his 

 fancy. D A F — G B E in that figure is the augmentation of 

 every semidiameter, while the altitude of the object continues 

 the same. But if Mr. M.'s statement on this subject and his 

 interpretation of the term parallax were correct, the diameter 

 parallel to the horizon would have no atigmentation, as the dif- 

 ference of the parallax of those '* two diametrically opposite 



limbs " 



