1826.] Biographical Account of Dr, Wilson, 327 



hydrostatical bubbles made the subject of a discourse delivered 

 by Mr. Wilson iii the winter of 1757. At this time he also 

 showed how a single glass bubble may serve for estimating very 

 small differences of specific gravity of fluids of the same kind, 

 such as water taken from different springs, or the like. This he 

 did by varying the temperature of such fluids, till the same 

 bubble, when immersed, became stationary at every trial, and 

 then expressing the differences of their specific gravity, by 

 degrees of the thermometer, the value of which can be computed 

 and stated in the usual manner. 



In the year 1758 he read another discourse to the same 

 society, upon the motion of pendulums. On this occasion he 

 exhibited a spring-clock of a small compass, which beat seconds 

 by means of a new pendulum he had contrived, upon the prin*- 

 ciple of the balance, whose centres of oscillation and motion 

 were very near to one another. At one of the trials, it performed 

 so well as not to vary more than a second in about forty hours, 

 when compared with a very exact astronomical clock near to 

 which it was placed. It was some view of rendering much 

 more simple and cheap the machinery of ordinary movements, 

 by the slow vibrations of such a pendulum, which induced Mr. 

 Wilson to prosecute these experiments. 



Not long after this, he also put in execution a remarkable 

 improvement of the thermometer, which consists in having the 

 capillary bore drawn very much of an elliptical form, instead of 

 being round. By this means the thread of quicksilver upon the 

 scale presents itself broad, and much more visible than it does in 

 a cylindrical bore of the same capacity. The difficulty of con- 

 structing thermometers of this kind had nearly hindered him 

 from completing his invention, as the thread of quicksilver was 

 found extremely hable to disunite, when descending suddenly in 

 so strait a channel. But, by his long experience, joined to 

 further investigation, and more trials, he at last discovered a 

 method of blowing and filling thermometers with flattened bores> 

 which freed them entirely from this defect. 



About the same time also, he conceived the design of con- 

 verting a thermometer graduated for the heat of boiling-water^ 

 into a Marine Barometer, in consequence of the well-known 

 difference of temperature which water, when boiling, acquires, 

 under the variable pressure of the atmosphere. This he effected 

 by making a boiling-water thermometer, about a foot in length, 

 with a pretty large ball, and having a thread of quicksilver as 

 broad and visible as was consistent with a very perceptible run 

 upon small alterations of temperature. The stem of this ther- 

 mometer he fortified, by inclosing it in a cylindrical case of 

 white iron, having soldered to it, at its lower end, a socket of 

 brass for receiving half of the ball, which afterwards becam© 

 entirely defended, by screwing to the socket a hemispherical 



