197 



diameter of the wide bore, which should be cylindrical, and neither 

 conical nor flat. 



4th. The tubes should be well deprived of air before being sealed. 

 The instrument may be thus graduated. 



If, when held vertically, the smaller tube being below, the mer- 

 cury at the ordinary temperature should fill the lower tube, the bulb 

 and part of the upper tube, the instrument may be pointed off in 

 the same manner as an ordinary thermometer. But, if the mercury 

 under these circumstances be not enough to fill the bulb, the best 

 plan is perhaps to lay the instrument horizontally in a vessel of 

 water, side by side with a standard thermometer, and, keeping the 

 extremity of the mercury in the one tube at a constant point, to 

 mark off its extremity in the other tube at two or more different 

 temperatures, as shown by the standard thermometer. The length 

 of this tube corresponding to a degree may he then found in the 

 usual way. 



The same process may be followed with the other tube. Or, 

 take two points in the first tube say A and B, the distance between 

 them being, say 50. Set the mercury at the point A, and mark off 

 its other extremity in the second tube. Set it now at the point B, 

 .and mark its extremity in the second tube. The distance between 

 these two points in the second tube will be the length corresponding 

 to 50. 



Graduate the tubes to within a short distance of the bulb, and the 

 best plan is perhaps to number the degrees from one extremity of 

 the instrument, beginning 0...10...20, &c., on to the mark on that 

 side nearest the bulb. Suppose this mark is numbered 100; then 

 number the mark nearest the bulb on the other side also 100, and 

 go on upwards numbering 110, 120, 130, &c., until reaching the 

 other extremity of the instrument. In the next place ascertain the 

 temperature of the mercury when it fills the bulb and reaches only 

 to the nearest mark on both sides. 



Let this be C. In taking an observation, note the numbers at 

 both extremities of the mercury, and deduct the less from the greater. 

 To the positive remainder add the constant C with its proper sian, 

 and the sum will give the true temperature. 



