Is atmospheric pressure exerts force from all directions of the object on the surface of the earth? This question came to me as I was solving problem asks the length of the fluid in a open tube when the tube makes particular angle with horizontal surface by giving the height of the fluid when tube was standing straight vertical.( considering atmospheric pressure). Do I have misunderstanding of atm. 
 A: Pressure is exerted by a fluid in directions perpendicular to the surfaces of objects which contains the fluid. 
To solve this problem you must understand that when the tube is tilted only a component of the mercury in the tube can exert a pressure along the length of the tube as it's a liquid. This is because the pressure from a liquid arises due to the gravitational force on it. Like when it's vertical the pressure is 8.0cmHg but when tilted 65 degrees to vertical, that is 25 degrees to horizontal then the component of pressure along the length of the tube is $8.0sin(25)$ cmHg.
The atmospheric pressure on the open end of tube along its length remains 76 cmHg regardless of angle as this pressure is not due to gravity( rather factors like temperature etc.) 
So the total pressure downward on sealed air column will be the pressure due to component of mercury($8.0sin(25)$ cmHg) and atmosphere(76 cmHg).
Then you can solve the problem with taking the length of sealed column of air being inversely proportional to the pressure exerted on it along the length of tube. This is because the longer the tube of air in a sealed tube the lower the pressure it can exert. Try to get the answer by this method.
