Edit: More on the Boston/Roxbury boundary can be found here.
From The Charter and Ordinances of the City of Boston together with the Acts of the Legislature Relating to the City, Boston, 1834.
An Act relative to the Boundary lines of the City of Boston and the Town of Brookline [passed February 22, 1825.]
Chapter 8-7 Section 1. Be it enacted by the Senate and House of Representatives in the General Court assembled, and by the authority of the same, That the agreement made by and between the Mayor and Aldermen of the City of Boston, for and in behalf of said City, and the Selectmen of the town of Brookline, in behalf of said town, be and the same hereby is ratified and confirmed, and that hence forth the bondary lines between the said city and town shall be as follows, viz. beginning at a point marked (a) on the plan drawn by S.P. Fuller, 1123 feet distant westerly from the westerly side of the filling sluices of the Boston and Roxbury mill dam; thence running northwesterly from the said point (a) at an angle of 115 degrees from the mill dam, until it strikes the centre of the channel of Charles River, and also running from the siad point (a) southerly, at an agle of 103 degrees forty minutes, until it strikes the centre of the channel of Muddy River, at a point where the respective boundaries of Boston, Brookline, and Roxbury meet each other.