140 Major A. W. Baird and Prof. G. H. Darwin. 



" Those in the Report of the B.A. are by Sir W. Thomson. In those 

 of the Coast and Geodetic Survey the A's (amplitude) correspond 

 with Sir W. Thomson's R's, but the e's (epochs) differ from his by 

 90° in the diurnal components. These corrections of his epochs I 

 introduced into my ' Tidal Researches' in 1874, p. 44, § 28.* 



" From a reference to Schedule I, Tides of Penobscot Bay, Professor 

 Darwin with reason concludes that I have not applied this correction 

 in the diurnal component of the /c-tides. This arises from the omis- 

 sion by oversight of a footnote to Schedule I, as follows : — 



" 1 For A, 1 read A 1 — \ir in the diurnal component of the K-tide.' 



" The corrections have, in all cases, been applied according to this 

 note. 



" Iu my ' Tidal Researches T of 1874 I have given formulae for the 

 correction of both the amplitudes and epochs depending upon the 

 position of the moon's node. These corrections reduce them to what 

 they would have been if the moon had moved in the ecliptic. By a 

 reverse method these amplitudes and epochs can be reduced back to 

 any year for which practical application of the results is required. 

 In the discussion of tides in Penobscot Bay I have also given small 

 tables, Tables III — YI inclusive, to facilitate these corrections, and 

 reductions depending upon the lunar node. The double signs, how- 

 ever, of Tables III, Y, and YE, got reversed somehow in copying and 

 printing ; but the signs have been used correctly in the reductions, 

 even in those of the Report in which the signs are given erroneously, 

 which shows that they were at first correct, and that the error was 

 introduced in copying. 



" These nodal corrections have in all cases been applied to the 

 results, so that in these corrected results the irregularity of long 

 period depending upon the moon's node is eliminated, and the ampli- 

 tudes and epochs are the same from year to year, except so far as 

 they are affected by small irregularities from abnormal disturbances 

 not completely eliminated. An exception to this, however, is the case 

 of the St. Thomas tides, in which the reductions were not carried so 

 far, and these small nodal corrections were not applied to these small 

 tides. The amplitudes and epochs are those simply belonging to 

 the years of observations. . . . It is certainly desirable to have 

 an international uniform notation. 



" I should have stated sooner that in Table II, column 0, 90° have 



* [Notwithstanding this assurance I venture to think that Mr. Ferrel must be 

 mistaken. For example, at Sandy Hook, it looks as though it were certain that 

 K 2 , L, X have been reduced according to one rule, and the rest of the semi-diurnal 

 tides according to another ; for the phases differ by about 180°. Compare again 

 O, Kj, P with J and Q at Penobscot Bay.— Gr. H. D., August 12, 1885.] 



[It may be noticed that k of S x for San Diego differs by 180° in the U.S. reduc- 

 tion from the value in the B.A. reduction. I have no evidence as to which is 

 correct.— 0-. H. D., October, 1885.] 



