54 



SCIENTIFIC RECREATIONS. 



is used instead of mercury, but as the latter is thirteen-and-half times 

 heavier than water, a much longer tube is necessary; viz., one about thirty- 

 five feet in length. The construction is easy enough. A leaden pipe can 

 be fixed against the house ; on the top is a funnel furnished with a stop- 

 cock, and placed in a vase of water. The lower part of the tube is bent, 

 and a glass cylinder attached, with another stop-cock- the glass being about 

 three feet long, and graduated. Fill the tube with water, shut the upper 

 stop-cock, and open the lower one. The vacuum will be formed in the top 

 of the tube, and the barometer will act on a larger scale than the mercury. 



The Glycerine Barometer, invented by Mr. Jordan, and in use at the 

 Times office, registers as more than one inch movements which on the mer- 



Fig. 52. The principle of the diving-bell. 



curial thermometer are only one-tenth of an inch, and so are very distinctly 

 visible. The specific gravity of pure glycerine is less than one-tenth that of 

 mercury, so the mean height of the glycerine column is twenty-seven feet at 

 sea level. The glycerine has, however, a tendency to absorb moisture from 

 the air, but Mr. Jordan, by putting some petroleum oil upon the glycerine, 

 neutralized that tendency, and the atmospheric pressure remains the same. 

 A full description of this instrument was given in the Times of 25th October, 

 1880. 



The uses of the barometer are various. It is employed to calculate the 

 heights of mountains ; for if a barometer at sea level stand at 30, it will 



