The following improvement on the construction of the thermometer, by which it is fitted to mark the lowest or the highest point to which the fluid has attained in the absence of the observer, is due to John Rutherford, M. D. of Middle Balilish. This gentleman communicated it to me some time ago, and accompanied the description with one of his thermometers. The contrivance is so very simple and ingenious, that it well deserves to be made public. I therefore, by permission of the author, beg leave to lay an account of it before the Royal Society.