Dr. Collihole was a member of the Order of Engineers in London.  He was first introduced in the book Fever Crumb, and was reported to be dead in the second book in the trilogy, A Web of Air, by Dr. Avery Teal.  He was an Engineer who believed flight was possible, and had constructed a hot-air ballon on the roof of Godshawk's Head.  Fever and Shrike used this balloon to escape from London rioters.  Later on in the book, he revealed that he had been in contact with the Nomad Empire, and made his way to another spy, only to be captured by London rioters looking for Engineers for Ted Swiney

In the next book it was revealed that it was Dr. Teal/Vishniak who had killed him, working for the Suppression Office, a new section of the Guild of Engineers constructed to ensure by any means that new technologies that would ive London's enemies an advantage could not develop.