METEOROLOGY AND CLIMATOLOGY. 
27 
the mercury in the barometer tube. These two figures he is to enter in 
his note-book, and unless he is himself discussing the results, he 
should apply no correction whatever to them. The rules for observing, 
then, are:— 
1. Read the attached thermometer and note the reading. 
2. Bring the surface of the mercury in the cistern into contact with 
the ivory point which forms the extremity of the measuring rod by 
turning the screw at the bottom of the cistern. 
The ivory point and its reflected image in the 
mercury should appear just to touch each other 
and form a double cone. 
3. Adjust the vernier scale so that its two 
lower edges shall form a tangent to the convex 
surface of the mercury. The front and back 
edges of the vernier, the top of the mercury, and 
the eye of the observer are then in the same 
straight line. 
4. Take the reading, and enter the observation 
as read without either correcting it to freezing- 
point or reducing it to the sea-level. 
The scale fixed to the barometer is divided 
into inches, tenths, and half-tenths, so that each 
division on this scale is equal to 0*050 inch. 
The small movable scale or vernier attached to 
the instrument enables the observer to take more 
accurate readings; it is moved by a rack and 
pinion. Twenty-four spaces on the fixed scale p ia> 5 . p wo Headings of 
correspond to twenty-five spaces on the vernier; the Barometer Vernier . 
hence each space on the fixed scale is larger than 
a space on the vernier by the twenty-fifth part of 0*050 inch, which is 0*002. 
Every long line on the vernier (marked 1, 2, 3, 4, and 5) thus corresponds 
to 0*010 inch. If the lower edge of the vernier coincides with a line on 
the fixed scale, and the upper edge with the twenty-fourth division of the 
latter higher up, the reading is at once supplied by the fixed scale as 
in A (Fig. 5), where it is 29*500 inches. If this coincidence does not 
take place, then read off the division on the fixed scale, above which the 
lower edge of the vernier stands. In B (Fig. 5) this is 29*750 inches. 
