VOL. L.] PHILOSOPHICAL TRANSACTIONS. 141 



large drops ; which would make the instrument less accurate. For the thermo- 

 meter's beginning to rise immediately after a drop is fallen, or just as it is going 

 to fall (in which case it will return back into the tube^) will make a difference 

 of such part of a degree nearly as that drop answers to. To prevent this incon- 

 venience, the top of the shorter leg close to the ball is contracted, by being 

 held in the flame of a lamp ; and the passage is further straitened by a solid 

 thread of glass placed within the tube, and extending from the bottom of the 

 shorter leg to the part near the ball a, where it is most contracted. By this 

 means, as soon as any small portion of mercury is got beyond the end of the 

 thread of glass, it breaks off and falls into the ball in very small drops. This 

 thread of glass is fastened by the heat given to the tube in making the bend 

 next to the ball. In order to fill the shorter leg with mercury, to fit the in- 

 strument for a new experiment, it must be inclined till the mercury in the ball 

 covers the orifice of the tube n. The cylinder being then heated, the mercury 

 will be forced into the shorter leg, and will run down the thread of glass in drops, 

 which will soon unite. By this means such a quantity of mercury must be got 

 into the shorter leg, as on the cooling of the instrument, will be sufficient to 

 drive all the spirit of wine into the ball with a less degree of cold than what the 

 thermometer is likely to be exposed to. The scale of degrees on the shorter 

 leg will, in different seasons, be liable to an error of the same kind as that 

 which was explained in the first-mentioned thermometer; but in this it will be 

 less considerable, as the space between the two scales is filled with mercury, 

 whose expansion is about (3 times less than that of spirit of wine. 



The thermometer for finding the greatest cold, if applied to the purpose of 

 immerging it deep in the sea to find its temperature, must be left open at top, 

 for fear of breaking by the pressure. There is another inconvenience to be 

 avoided ; which is, that the mercury in the ball a, by the tossing of the instru- 

 ment, might sometimes get into the shorter leg of the syphon, which would 

 spoil the experiment. To prevent such an accident, the most convenient 

 construction which occurred, was that of fig. 4, which differs from fig. 3, in 

 having the ball a omitted ; so that the mercury running out of the shorter 

 leg will fall to the bottom of the cylinder, and will not be so liable to get back 

 into the tube by motion. The cylinder is made to stand not quite parallel to 

 the legs of the syphon, that the contained mercury may more easily be brought 

 to touch the end of the tube, in order to fit the instrument for a new experi- 

 ment. If by means of a bladder the sea-water can be kept out of the glass, 

 this instrument may be made to show the common degrees of heat. If ther- 

 mometers of this kind were to be sent up into the air by means of a kite, they 

 might be made like those proposed for the sea ; but it would not be necessary to 

 leave them open. 



