Letter, from Henry Oldenburg to Renatus Slusius, dated at London Thanks for welcoming English travellers who have passed through Liege; refers Slusius to the works of John Wallis and of James Gregory who have both demonstrated Guldin's centrobaric rule; recounts literary notices from Mr Collins, who also sends some mathematical problems for Slusius; sends Isaac Barrow's attempt at solving the problem with the two conic sections 25 September 1673