Letter from Henry Kater, 10 Great Castle Street, to an unknown correspondent The two circles in his correspondent's astronomical circle furnish a means of obtaining latitude by two altitudes of a heavenly body with the interval of time between them by means of the change os azimuth. He encloses a method [not present] although he thinks the suggestion might already have occurred. He is obliged for a message relayed by Captain Grover and is sorry to hear that his correspondent is unwell. no date