518 Division of Astronomical Instruments. [Sept. 



Cotidal Lines which I saw with much pleasure. But I am desirous of removing a 

 misapprehension which I perceive in the remarks accompanying this table. Mr. 

 Sinclair has given the time of high water for every day of the moon's age, at the 

 places contained in his table, calculated on the supposition of a daily retardation 

 of the tide, to the amount of 48 minutes : and in the remarks a rule is quoted from 

 my paper for the correction of the time so given. But the rule quoted is errone- 

 ous for the purpose there stated. The rule which should have been given is the 

 following nearly. 



Correction to be applied to the time of high water calculated by supposing 

 it to be always at the same interval after the moon's transit as it is on the days 

 of new and full moon. hours. 



Time of moon's preceding transit, 1 23456789 10 11 



Correction, minutes, 0—16—31 — 46—61 — 72—75—65—34 o-f- 13 + 11 



The fact is, that the correction quoted from my paper belongs theoretically to 

 the " correct establishment," or mean of all the intervals of moon's transit and 

 tide, not to the " vulgar establishment" or interval of moon's travel and tide on 

 the day of new and full moon, which is the establishment taken by Mr. Sin- 

 clair. 



The correction which I have given above is probably not exact for India, for 

 it is taken from the London Tide Observations ; and it would be extremely 

 desirable, as you have observed in your Journal, to verify or correct it by obser- 

 vations at some stations in the Indian seas, made daily for a sufficient length of 

 time. I may add, that the above correction is what has been called the semi- 

 menstrual inequality, and does not arise from the inequality of the moon's daily 

 motion, but from the varying angular distance of the moon from the sun, in con- 

 sequence of which the solar tide sometimes coincides with the lunar, and at other 

 times is separated from it by a large angle. 



I am very glad to find you expressing your hopes that you will be furnish- 

 ed by your correspondents with tide observations from an extensive range of 

 places in India. I would observe, that for the purposes of science, the daily ob- 

 servations themselves are much more valuable than the "establishment," or any 

 other inference collected from them. 



In conclusion, I would beg particularly to state, that directions have been 

 given for tide observations on the whole coast of England from the 9th to the 

 22nd of June in this year: that I have strong hopes that these observations will 

 also take place on the shores of other states of Europe and America, at the same 

 time; and that it would be very interesting and useful to have contemporary 

 observations made on the shores of India at as many places as possible." 



March 21, 1835. 



2. — Mr. Taylor's mode of determining the Errors of Division in Astronomical 



Instruments. 

 [Extract of a letter from Sir John Herschel, dated C. G. H. April 20, 1835.] 



Feldhausen, near Wanbey, C. G. H. 



"The Journal for August, 1834, contains Mr. Taylor's application of the 



collimating principle to the examination of the mural circle at Madras. It is 



somewhat singular, that not long before I had suggested to Mr. Maclean, the 



Astronomer Royal heve, and had also written to Professor Airy at Cambridge, 



