New York. January 29. 1753
About a week ago the papers whereof that
herewith Mark’t A is a Translation were sent to Peruse, and on my
returning them With a Translation I beged Leave to Communicate
Coppies of them to You and Dr. Colden with Liberty if you thought
proper to print them or any Part of them. In Answer To which I
obtained the Leave coppied at the end of Them and Struck out La
Gallissonieres name, for the reason in the Leave.
The Delay of that Leave put me upon Dipping
into the Abridgment of the Philosophical Transactions to see what
[was] there said of the matters in those papers, and from thence I
made the Extracts in the paper Herewith Mark’t B. by which I find
that that which had baffled all the Art of man hitherto To discover
with any Tolerable Certainty (Vizt. the Suns Distance from the
Earth) may with great Certainty be Discovered by the Transit of
Venus over the sun the 26 of May 1761 Old Stile, if well Observed
in the East Indies and here and these Observations Compared
Together.
It Would be a great honour To our young
Colledges in America if they forthwith prepared themselves with a
proper apparatus for that Observation and made it. Which I Doubt
not they would Severally Do if they were Severally put in mind of
it and of the great Importance that that Observation would be To
Astronomy and that the missing that One Observation cannot be
retrieved for 250 years To come.
You have on so many Occasions Demonstrated Your
Love To Literature and the good of Mankind in General that I
thought no person so proper as your self to think of the ways and
means of perswadeing these Colledges to prepare themselves for
taking that Observation and in order to it you may make what use
you please of the papers herewith, only not my name.
Endorsed: Coppy Letter to Mr Franklin Janry 29 1753
What Put me upon giveing you the Trouble of
this Was the Sight which a few Days agoe I had of some Letters from
the Royall Academy of Sciences at Paris with directions for
Observing in Canada the End of a Transit of Mercury over the sun
Which is To happen on the 6. of may Next and by their Estimation
about ¾ past five in the morning at Quebeck and that it Ends at or
possibly before sun riseing at the Lac Superieur.
This made me Look into Lowthorpes Abridgment of
the Transactions of the Royall Society at London page 431. and by
that in the Longitude of 5° West of London, (Which I suppose is
nearly the Longitude of Phileadelphia as New York by many
Observations of the Eclipses of Jupiters first Satellite Was found
in 4h. 56’ nearly) it would seem To me that the Transit would End
three minutes after Five in the morning, but if it does, yet both
Interior and Exterior Contacts at the end, may be seen at
Phileadelphia and here the transition may be Seen some minutes
before the Contacts, or yet Still Longer in Connecticut, Boston,
Halifax, and Louisbourgh by reason of their Greater Latitudes and
Less Longitudes.
The Jesuits in China and the East Indies I
doubt not will Observe it in Sundry places there as the Whole
Continuance of Mercury on the sun will be there Visible and if well
Observed there and the End well Observed in North America it will
bid the fairest of anything hitherto had To give the Suns parrallax
or distance from the Earth, for the Triangles made of the
Observations in the East Indies With those here will have known
Bases of above ¾ of the Diameter of the Earth; the Next Transit by
page 432 is in 1756 but It will not be visible here.
But the Critical Time for Obtaining the suns
Parrallax will Be the Transit of Venus over the sun (by page 436)
the 26 of May 1761 Old Stile which in 5h. west Longitude I Esteem
will end at 53’ after 4 in the Morning and be visible at
Phileadelphia for ¼ of an hour after Sun rise and much Longer
Northward and Eastward.
Jones’s Abridgment of the Transactions, Vol. 4
page 213 &ca. has a Discourse of Edmund Halley concerning that
transit of Venus and a figure of it and by good Observations of
that, he Says, the parrallax of the sun may be Ascertained to
whereas I doubt of it’s Being Certain now To ¼ for Halley there
Supposes it May be 12½ Seconds Wheras Flamstead in Sundry places of
the Transactions from his Observations of Mars when nearest the
Earth Esteems it not To Exceed 10'’.
By Transactions Vol: I, 435 and 436 you’ll see
that No Transit of Venus till the year 2004 Will be visible both
here and in the East Indies at the Same time so that if this
Opportunity be missed there Will be None so good for 250 Years To
Come To Ascertain this Much Desired points of the Suns parrallax
which being Known The parrallax of all the planets is of Course
from thence found.
As there are Now Sundry Nurseries of Learning
Springing Up in Pensilvania, Jersey, New York, Connecticut, and
Boston; all ways Should be thought of To Induce each of those To
provide a proper apperatus for makeing such Observations, Long
before the year 1761 that they May be Expert at Takeing
Observations of the Kind before that Transit Happens; and in the
mean time To be Ascertaining the Longitudes of these places by
Observations of the Eclipses of Jupiters first Satellite and the
Latitudes of the places by good Quadrants for these things should
Be Known and Ascertained to Render the Observations of the Transit
of use for Ascertaining The parrallax.
Another Reason why preparation should be made
in all these places for that Rare Event, is; that should it happen
to be Cloudy in some of these places yet it may be Clear in Others
and by means of them that have Clear weather the Great End may be
Gained; that Event may Well be Called Rare as Horrox who says the
Last Transit of Venus over the sun On November 24 1639 now 114
years ago is the Only one of the Posterity of Adam who has seen one
of these Transits.
I have said before that the suns Parrallax is
not Ascertained To ¼ part but it seems there is a much greater
Difference in Opinion about it than that, for tho’ it be Generally
Taken now to be 10’ yet the Book called Matho or the Cosmotheoria
puerilis printed in 1740 Vol: 2 page 388 &ca. gives very
plausible Reasons why it Cannot be Less than 20'’ and he thinks
26'’ a more Reasonable Number. For he shows that if the
parrallax be Only 10'’ then the point of Equall Attraction
Between the Sun and Earth, is but 45 Semidiameters of the Earth
Distant from the Earth but the moons Distance is about 60 and
therefore 15 SemiDiameters in it’s Conjunction Nearer the Sun than
the point of Equall Attraction and from thence he Concludes that as
there the Attraction is greater To the sun than to the Earth it
Could Never Adhere To the Earth but be Drawn from it Towards the
sun and allowing the parrallax to be 20'’ yet even in that Case
the moon Would be sometimes Beyond the point of Equal attraction
and 26'’ Should be allowed To Keep it always within that Point,
which Reasons prevailed with me to be of his Opinion Till I Saw Mr.
Maclaurins Explanation of Sir Isaac Newtons philosophy which
Demonstrates Clearly that tho’ the moon Goes Without the point of
Equall Attraction as Matho says, Yet it Will Adhere To the Earth by
Means of its projectile free in its Orbit Round the Sun.
To Point out what apparatus of Instruments
should be procured I Know of Nothing Better than To referr to the
Abridgments of the Philosophical Transactions where multitudes of
Observations of the Heavenly Bodys appear and the Instruments they
were Observed with and how, And as To the Observations of the
Transit of Mercury in particular and Instruments the Observers made
use of, You’ll see Largely in the Abridgment of Philosophick
Transactions Vol: 8 page 194 To 208 and Vol: 4. 213 &ca. before
mentioned.
Each of these places which have not a Set of
the Abridgments of the Philosophick Transactions should in the
first place Get them Being in 9 Vol: quarto which as they will be
Cheaper so will they be Much Easier To Turn To, than the
Transactions themselves by Means of their Tables.
I find by the preface To Halleys Tables page 3
that No. 386 of the Transactions Contains Halleys Corrections of
his Numbers of Mercury. But that Volume 6 Containing that Number I
have Lent To a Gentleman Living at 70 miles Distance from here.
I Find by Halleys Tables that Macao in China is
in 7h. 35’ and Pekin in China in 7h. 45’ East Longitude yet by
reason of their Northern Latitudes and the suns Northern
Declination at the times of Both the preceeding Transits they may
see the ends of them Before the Sun Setts in those places.
I have Computed what would be the Lengths of
the Bases of triangles made of Observations at Phileadelphia and
Observations in the following places in the East Indies, and found
them as follows
On the papers of Directions herewith Mark’t A I
Would Observe that I think them very good and that I find all or
most of them in the Transactions and that if the Observer is
acquainted With makeing Observations of that Kind they Seem Rather
Too particular. If he is not acquainted, then the Neglect of
Mentioning the Smoaking the Eyeglass before he Looks at the Sun
thro’ the Telescope and Quadrant (if they have not Smoak’t plain
Glasses fitted without the Eyeglass) may run the Observer into the
Danger of Looseing an Eye. If he is to Observe the figure of the
Sun Cast on a paper thro’ the Telescope then the Darkning of the
Room should have Been mentioned.