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Soho Square {September} 1 1787
Sir
I have received your ingenious Letter relative to the variation of the Needle, & take the liberty of advising you to pursue with Diligence the Subject on which it appears to me you have made a progress sufficient to authorize a reasonable hope that science will derive real increase from your labors.
The Royal Society having lately removd into a new House, the first Series of observations relative to the variation is only now in its Course. I cannot therefore tell you with the utmost precision what the variation is there; our Instrument [text illegible due to paper fold] which probably is sufficintly exact for your purpose ~ Presently when the Instrument is moved, we shall find if the Magnetism of the Building has any material effect upon it of which if it has I shall with pleasure inform you.
I am Sir your obedient & most {Humble} Servant
{Joseph} Banks
Mr. John Churchman Philadelphia