Copy letter, from Sir John Herschel to William Henry Sykes, dated at Collingwood Copy of letter, copied as part of the correspondence project led by Colonel John Herschel FRS following the death of his father. Notes that actinometer observations prove faulty due to problem with instruments. Asks that William Henry Sykes notify Indian observatories to join others in terminating such observations until John Frederick William Herschel finds method of getting around problem. 11 March 1847