346 On Jindhig the Longihide hj Lunar Olservaiions. 



four-footed progeny on me ? If it be true of two limbs, it is no 

 concern of mine though it were false of fifty. 



Indeed the only shadow of objection that can be brought against 

 my statement is, that the bounding circle of the lunar disk is 

 rather nearer the observer than the moon's centre is. But such 

 an objection, which he was not likely ever to think of, would be 

 perfectly ridiculous. Wc might next talk of the effect of a lunar 

 volcano. Laying then that frivolous objection out of the way, I 

 am fully warranted to repeat, that the difference of the parallaxes 

 of any tiuo diametrically opposite points is the augmentation of 

 that diameter. I say, that diameter itselfj because the augmen- 

 tation is not the same for every diameter. For I must take the 

 liberty of informing one who arrogates to himself so much supe- 

 riority over me, that although, as I said before, the apparent disk 

 of the moon is not rendered elliptical by parallax ; yet when every 

 bounding point of that disk has been corrected for parallax, the 

 resulting figure is an ellipsis whose transverse axis is parallel to 

 the horizon. This will be clear to everv unprejudiced person, 

 when he considers that the bounding circle of the apparent disk 

 as seen from the earth's surface, is an ellipsis when viewed ob- 

 liquely from the earth's centre : and it is with this one circle that 

 we have to do. 



The correction called the augmentation is therefore greatest 

 in a vertical direction, that even existing at the horizon in a cer- 

 tain sense ; and on this principle, perhaps, might the augmenta- 

 tion be computed, more free from theoretical objection than any 

 other equally simple. Mr. R. is not likelv to relish this doctrine. 

 He will certainly dismiss it with a sneer of sovereign contempt, 

 as he kindly did the '^ angular point of the triangle ;" because he 

 could not offer one mouthful of rational argument against it. 



He would be thought, no doubt, to construct the figure on 

 page 248, as if he had viewed the moon from the earth's centre. 

 But will any one who looks at that most absurd diagram, believe 

 that Mr. Riddle was any lower than the bottom of a Newcastle 

 coal-mine, where he would be infinitely less in danger of a fall 

 than if hovering in the air " over London-bridge "? 



With regard to Dr. Mackay, and some others, not having no- 

 ticed the first-mentioned defect of the correction of latitude, we 

 have no evidence to the contrary. But in order to show that even 

 Mackay himself, whom I nevertheless highly esteem, was liable 

 to similar mistakes, (and who is not?) I beg to refer the reader 

 to page 353, vol. i. of his Treatise on the Longitude, where he 

 "vvill find a very erroneous rule for a correction of the azimuth, 

 arising from the change of declination during the time elapsed 

 between observed equal altitudes. The very same thing, illustrated 

 by an example, occurs in his Navigation. Others perhaps have 



copied 



