460 On a New Barometer, called " the Amphisbama." 



of quicksilver, not exactly 10 inches or 50 centimetres, but 

 such a length as would be 10 inches or 50 centimetres at 

 zero of temperature Centigrade. Then, by taking the simple 

 number for use in the calculation, whatever the real length 

 may, on account of temperature, be, the final result is the 

 reduced barometer. 



A convenient length for a portable instrument would be 

 about 20 inches, with a 10-inch thread of quicksilver. 



The diameter of the tube should be small enough to main- 

 tain continuity in the quicksilver. 1*2 millim. or one 

 twentieth of an inch answers very well. 



The value of an observation made with this instrument 

 would theoretically not be injured should the quicksilver 

 thread become, through mischance, discontinuous, so long as 

 all the liquid remained in the tube, though it appears that 

 practically the multiplication of ends by subdivision adds a 

 certain amount of friction, and produces sluggishness in the 

 movements of the quicksilver. 



In using the instrument for determining mountain eleva- 

 tions, the length of the quicksilver may be left quite unde- 

 termined ; for the difference of level appears as a function of 

 the ratio merely of the two pressures at the points of observa- 

 tion ( <§ log —J, where § is the height of the homogeneous 

 atmosphere. Therefore we can use the ratio of the func- 

 tions ( — — :~ I of the observations only at the two stations. 



In practice the glass tube is sunk into a groove in a light 

 piece of wood provided with two eyes, which serve either for 

 suspension from a nail in a wall, or for attaching a strap or 

 cord for slinging the instrument round the shoulders in 

 climbing. The eye at the open end of the tube will screw 

 in far enough to stop the opening temporarily if that should 

 be desirable ; but if care is taken to climb with the closed 

 end downwards, there is small fear of the quicksilver becoming 

 either lost or discontinuous. 



The instrument, as made by Messrs. Watson Brothers, of 

 4 Pall Mall, weighs between 6 and 7 oz. 



