Washington Oct. 19. 08.
Your two letters of the 11th. inst. have been recieved, & I am obliged to observe that so wholly do the indispensable duties of my office engross my whole time that I could not give a deliberate reading to two letters so voluminous as these & not relating to my particular functions, without withdrawing time from objects having stricter claims on me. I have run over them hastily and percieve that you are still engaged in the pursuit of the method of finding the longitude at sea by an observation of Jupiter & his satellites brought to the horizon by a double reflection as in Hadley’s quadrant, that you have written a play to raise funds for prosecuting this, & wish me to circulate a subscription for it & print your letters, I will willingly subscribe myself for a number of copies to help you, but I have never permitted myself to be the circulator of any subscription or to have agency in printing any thing, concieving it improper in my present office. and however wishful of your success in raising funds, I confess I should think them better applied to the comfort of your family. after so many better opinions it may be superfluous to offer mine. yet justified by my friendly motives in doing so, I will observe that to get the longitude at sea by observation of the eclipses of Jupiter’s satellites two desiderata are wanting. 1. A practicable way of keeping the planet & Satellite in the field of a glass magnifying sufficiently to shew the Satellites. 2 a time piece which will give the instant of time with sufficient accuracy to be useful. the bringing the planet & Satellite to the horizon does not sensibly facilitate the observation, because the planet in his ascending & descending course is at such heights as admit the direct observation with entire convenience. on the other hand, so much light is lost by the double reflection as to dim the objects & lessen the precision with which the moment of ingress or egress may be marked. this double reflection also introduces a new source of error from the inaccuracy of the instrument. 2. the desideratum of a timepiece which notwithstanding the motion of the ship shall keep time during a whole voyage with sufficient accuracy for these observations has not yet been supplied. fine time keepers have been invented, but not equal to what is requisite, all of them deriving their motion from a spring, & not from a pendulum. Indeed these pursuits have lost much of their consequence since the improvement of the Lunar tables has given the motion of the moon so accurately as to make that a foundation for estimating the longitude by her relative position at a given moment with the sun or fixed stars—every Captain of a ship now understands the method of taking these lunar observations & of calculating his longitude by them. I have gone into these details with the most friendly view of dissuading you from wasting time, which you represent as so much needed for your family, in a pursuit which has baffled every human endeavor as yet, & has lost so much of it’s importance. I return you your letters because you wish to have them published, & conclude with my best wishes for the success of your endeavor to raise the funds you desire, and for that application of them which shall be best for yourself & your family.
DLC: Papers of Thomas Jefferson.