SIR,--I duly received your letter of October 24 on the publication of an Ephemeris. I have long thought it desirable that something of that kind should be published in the United States, holding a middle station between the nautical and the common popular almanacs. It would certainly be acceptable to a numerous and respectable description of our fellow citizens, who, without undertaking the higher astronomical operations, for which the former is calculated, yet occasionally wish for information beyond the scope of the common almanacs. What you propose to insert in your Ephemeris is very well so far. But I think you might give it more of the character desired by the addition of some other articles, which would not enlarge it more than a leaf or two. For instance, the equation of time is essential to the regulation of our clocks and watches, and would only add a narrow column to your 2d page. The sun's declination is often desirable, and would add but another narrow column to the same page. This last would be the more useful as an element for obtaining the rising and setting of the sun, in every part of the United States; for your Ephemeris will, I suppose, give it only for a particular parallel, as of New York, which would in a great measure restrain its circulation to that parallel. But the sun's declination would enable every one to calculate sunrise for himself, with scarcely more trouble than taking it from an Almanac. If you would add at the end of the work a formula for that calculation, as, for example, that for Delalande, § 1026, a little altered. Thus, to the Logarithmic tangent of the latitude (a constant number) add the Log. tangent of the sun's declination; taking 10 from the Index, the remainder is the line of an arch which, turned into time and added to 6 hours, gives sunrise for the winter half and sunset for the summer half of the year, to which may be added 3 lines only from the table of refractions, § 1028, or, to save even this trouble, and give the calculation ready made for every parallel, print a table of semi-diurnal arches, ranging the latitudes from 35° to 45° in a line at top and the degrees of declination in a vertical line on the left, and stating, in the line of the declination, the semi-diurnal arch for each degree of latitude, so that every one knowing the latitude of his place and the declination of the day, would find his sunrise or his sunset where their horizontal and vertical lines meet. This table is to be found in many astronomical books, as, for instance, in Wakeley's Mariner's Compass Rectified, and more accurately in the Connoissance des tems, for 1788. It would not occupy more than two pages at the end of the work, and would render it an almanac for every part of the United States.