The mystery is now solved by giving here the letter in full. It is even more important than Dr Zach with all his enthusiasm supposed. It is not, however, from the pen of Northumberland, though none the less interesting on that account. The letter is in the well-known handwriting of Lower, of Tra’venti, on Mount Martin, near Llanfihangel, in South Wales, to his dearly loved friend and master Hariot at Sion, and is dated the 6th of February, 1610. The letter fills two sheets of foolscap paper. The first sheet of four pages Dr Zach found at Petworth, and it is to be hoped that it still exists there. The other sheet of four pages is preserved in the British Museum (Add. 6789). How long these two sheets have been separated it is difficult to tell, but probably from Hariot’s day, that is, for more than two centuries and a half. The two fragments are now brought together and printed for the first time complete, the first half from Dr Zach’s text, and the latter half copied verbatim direct from the original autograph manuscript, Brit. Mus. Add. 6789. LETTER FROM SIR WILLIAM LOWER MATHEMATICIAN AND ASTRONOMER TO THOMAS HARIOT AT SION FEBRUARY 6, 1610. I have receeved the perspective Cylinder that you promised me and am sorrie that my man gave you not more warning, that I might have had also the 2 or 3 more that you mentioned to chuse for me. Hence forward he shall have order to attend you better and to defray the charge of this and others, that he forgot to pay the worke man. According as you wished I have observed the Mone in all his changes. In the new I discover manifestlie the earthshine, a little before the Dichotomic, that spot which reprefents unto me the Man in the Moone (but without a head) is first to be feene. a little after neare the brimme of the gibbous parts towards the upper corner appeare luminous parts like starres much brighter then the rest and the whole brimme along, lookes like unto the Description of Coasts in the dutch bookes of voyages, in the full she appeares like a tarte that my Cooke made me the last Weeke. here a vaine of bright stuffe, and there of darke, and so consufedlie al over. I muft confesse I can see none of this without my cylinder. Yet an ingenious younge man that accompanies me here often, and loves you, and these studies much, sees manie of these things even without the helpe of the instrument, but with it sees them most plainielie. I meane the younge Mr. Protherbe.