Gives CD the results of some calculations for "dip" over different distances, as requested.
My dear Darwin
Not having any books of tables with me here and not being able
to procure such a thing in This Town I was on the point of writing to you to say I could
not send you what you wanted, till I sent to Falmouth for my tables, when fortunately I
found some of my old work in my desk which gave the Dip for different distances and as
that increases as the square of the distance increases I was able to proportion
for the distances you wanted, and this led me to a very easy rule for finding it for
any distance which as it may be useful to you I will give you: it is merely to multiply the dip for one mile by the
square of the number of miles you want it for— not having tables I cannot
calculate the dip for one mile to any very great nicety but the mean of those dips I
have worked, gives 0,758 of a foot for a mile of distance, and that will be
near enough for your distances as it it is so near as not to make a foot error in
15 miles distance— in case I may not have explained the way of
working it I will give you an example.
With respect to refraction —I cannot now give you
the amount with any certainty—but it will be very trifling & for short
distances almost nothing, the usual allowance is
Now I have scribbled all this I am not at all satisfied that it is plain I am a poor hand at explaining things but if you cannot make it out write and ask more questions—
I am very sorry to hear the account of your health but I hope you will give up all work and rusticate till you are quite restored— recollect everything is inferior to health and all the success & fame in the world will never compensate for a weak stomach and its attendant evils I still feel that tho I am better now. report says a promotion will come out at the Queens Birthday, this makes me anxious tho I think, I cannot be passed over if it does come out. if I am I shall care for nothing more in connection with the service— in fact now I am almost sick of it I cannot bear to think of our voyage. tho that is not caused by Deferred promotion; it is by unmerited reproaches where I least should have expected them—but I must not [ touch] on this.
my wife joins me in remembrances to Mrs Darwin | God Bless you & Believe me your sincere friend | B J Sulivan
PS | You directed you letter to Falmouth which I have left this two months—
I have called the curvature Dip though I think it is not a right term
- f1 566.f1Sulivan is not talking about ‘dip’ in the usual nautical sense, i.e., the angular difference between the real and apparent horizon due to the observer's elevation above sea level. Rather he is referring to a correction to account for the curvature of the earth in measuring the elevation of a distant object. The generally accepted correction in terms of statute miles and feet is two-thirds of the distance squared (Close 1905, p. 81). It is not clear why Darwin needed the information.
- f2 566.f2A correction to offset the bending of light in the earth's atmosphere. The amount of refraction varies considerably depending on conditions, but the distance at which an object of a certain height can be seen is increased by roughly
th (Close 1905, p. 81). 1 13
- f3 566.f3Sulivan was not promoted to the rank of Commander until May 1841.