No - the direction relative to the spirit level does change. That's the whole point.
The direction is always "up".
When the level is at 45 degree angle, the low end is accelerating up at 9.80665 m/s2 and the high end is accelerating up 9.80665 m/s2. The low end is accelerating in the same direction and subject to the same amount of force as the high end and the bubble is not level.
When the level is at 90 degree angle the left side is accelerating up at 9.80665 m/s2 and the right side is accelerating up at 9.80665 m/s2. The left side and the right side are accelerating in the same direction and subject to the same amount force. Why is the bubble in the middle at a 90 degree angle and not a 45 degree angle when in both cases, they are accelerating in the same direction and subject to the same amount of force?
Your assertion is that "an external force" is responsible instead. Which force, specifically? Name it, identify its source, and give us an idea of the magnitude and direction.
It could be any force....the wind or whatever, but in general it would be somebody physically moving it...see my references below.
Your source also does a fantastic job of explaining why the line you were discussing cannot be straight - despite you insisting that it was the very definition of "straight".
Ok, let's think about it for a minute. I think we can safely assume the source would agree that the earth is round and I think we can also safely assume that the source would agree that a level is straight.
The source clearly says that "level" means that all points are equal distance from gravity, so it would follow that the source would also agree that a straight level can indicate all points an equal distance from gravity on a round earth. It wouldn't make much sense for a surveyor to use a spirit level if that surveyor didn't believe that a straight level could indicate all points are equal distance from gravity if the earth is round.
In RET, gravity is what moves the bubble once the level is stationary. The problem is precisely that you keep claiming the opposite.
No, it isn’t the RET position that what moves the bubble once the level is stationary is gravity. I have already offered several references to prove that, but here are some of the more clear ones again.
When the left side is too high the bubble will sit on the right side of the graduation marks and vice-versa for the opposite side. Using this as a guide, you can now make the necessary adjustment to create a perfectly level project.
Translation: In order for the level the bubble, you have to adjust the level
https://www.home-dzine.co.za/diy/diy-spiritlevel.htm If the bubble floats to the left, raise the right side, and vice-versa.
Translation: In order to move the bubble, you have to manually raise or lower the level.
https://everything2.com/title/Spirit+level To counter this effect the end of the level nearer the mass must be tipped up to recenter the bubble. In other words, the level surface tips upward as one approaches the mass.
Translation: You have to tip the level in order to center the bubble.
https://history.nasa.gov/SP-4211/ch11-4.htm For example, if a bubble is off to the right, then your surface rises to the right. Conversely, that same surface could be described as sloping down to the left. It’s a matter of perspective. Adjust your area or the object as necessary and repeat until you get a level reading you desire.
Translation: You have to adjust the object you are leveling relative to the level for the bubble to read level
https://www.keson.com/how-to-read-a-spirit-level/ Now, looking at the vial which is at right angle to the base of the spirit level – if the bubble is between the marks, the surface is vertical. If the bubble sits outside the marks, move the level until the bubble is centrally placed.
That one really shouldn’t need any translation.
https://www.actavodirect.com/blog/how-to-use-spirit-levels/Neither should the next one.
Where the bubble sits over or out of the graduation marks, adjust the spirit level to the left or right.
https://www.property24.com/articles/how-to-use-a-spirit-level/17759Let me know if these are not enough.