GREEN'S STANDARD BAROMETER. 



59 



is cut off; when too high, the light is seen above the top of the 

 mercury. It is right when the light is just cut off from the sum- 

 mit, the edge making a tangent to the curve. A piece of white 

 paper placed behind, and also at the cistern, will be found to 

 give a more agreeable light by day, and is, besides, necessary for 

 night observations ; the lamp being placed before the instrument 

 and above the eye, to reflect the light. 



Fig 



Pig. 5. 



30,000 



The method of reading off will perhaps be best explained by 

 a few examples. Suppose, after completing the adjustments, the 

 scale and vernier to be in the position shown in Fig. 4, on this 

 page, it will be seen that the lowest or index line of the vernier 

 coincides exactly with the line marked 30 on the scale. The 

 reading, therefore, is 30.000 inches. 



