times he was occupied with Chaillet, the Swiss dyer, experimenting on various chemical substances; corresponding with Dr. Black as to the new fluoric or spar acid; and at another time making experiments to ascertain the heats at which water boils at every inch of mercury from vacuo to air. Later we find him inventing a prismatic micrometer for measuring distances, which he described in considerable detail in his letters to Small.[89] He was at the same time busy inventing and constructing a new surveying quadrant by reflection, and making improvements in barometers and hygrometers. “I should like to know,” he wrote to Small, “the principles of _your_ barometer: De Luc’s hygrometer is nonsense. _Probavi._” Another of his contrivances was his dividing-screw, for dividing an inch accurately into 1000 equal parts. He states that he found this screw exceedingly useful, as it saved him much needless compass-work, and, moreover, enabled him to divide lines into the ordinates of any curve whatsoever. [88] The telescope was mounted with two parallel horizontal hairs in the focus of the eyeglass, crossed by one perpendicular hair. The measuring pole was divided into feet and inches, so that, wrote Watt, “if the hairs comprehend one foot at one chain distance, they will comprehend ten feet at ten chains,” and so on. This invention Watt made in 1770, and used the telescope in his various surveys. Eight years later, in 1778, the Society of Arts awarded to a Mr. Green a premium for precisely the same invention. [89] Letter to Small, 24th Nov. 1772. Watt, however, took no steps to bring this invention before the public, and in 1777, a similar instrument having been invented by Dr. Maskelyne, was presented by him to the Royal Society. Thus Watt also lost the credit of this invention.