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Jim,

After I posted, I realized I had not answered your question.  I can
only speculate that Sumner was the first to draw a line of position
through the two points.  But I don't know the meaning of the term,
double latitudes.

Fred

On Feb 28, 2005, at 8:35 AM, Jim Thompson wrote:

> Thanks Fred.  I understand Sumner's method very well, and the
> connection to
> Saint Hilaire's 30 years later, but I don't understand all the factors
> that
> lead up to Sumner's discovery of his method.  The "Double Latitude"
> concept
> obviously had been floating around already in his time, and may have
> given
> him the basic tools that allowed him to develop his approach.  If
> that's
> what I think it is, then maybe the idea of using one sight to
> calculate two
> solutions based on two assumed latitudes (one from DR, and one
> theoretical)
> was not that novel, in his time.  What was novel was his application
> of the
> procedure.
>
> Jim Thompson
> jim2@jimthompson.net
> www.jimthompson.net
> --------------------
> Outgoing email scanned by Norton Antivirus
>
>> -----Original Message-----
>> From: Fred Hebard
>> Sumner didn't use St. Hilaire's method, assuming one location and then
>> finding a line of position based on azimuth and distance from that
>> position.  Rather, he assumed two latitudes and found the longitudes
>> corresponding to those latitudes.  Then fitted a line of position
>> through those two locations.
>>
>> Having the assumed latitude plus the sight, he determined longitude.
>> Assuming a second latitude gave him a second longitude, with the same
>> sight data.
>>
>> Fred
>>
>>
>

   

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