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A level is simply a device for determining if a surface is “level” ie horizontal.  To a builder something is often level if it is level simply along, say, its length. But for an engineer for a surface to be level if has got to be level in two directions at an right angle to each other. This, though, assumes the surface is flat. If the surface is twisted it might be level in two directions but only at certain places.

Consequently a level can determine if a line on a surface is level or if a flat surface is level, or if a surface is not flat, in the sense that it is twisted.

If two lines in the same vertical plane are both level then they are parallel.

Two surfaces that are horizontal in all directions are also parallel to each other

Traditional levels consist of a glass tube that has a curved surface internally. this is filled with a liquid but with some air and is sealed. When the level is horizontal the bubble will rest between some marks scribed on the surface of the glass tube.


The key feature of any level is its sensitivity. This is how small a difference from the horizontal can be detected.

Builder’s levels

The most common sort of level is that used by builders. The sensitivity of these is about 1mm per metre.

With this sort of level if it is held in what appears to be a vertical position then this is good enough.

Engineer’s levels

Levels suitable for levelling machines are often called “engineer’s levels”.

Levels used for levelling machines are of the order of ten times this sensitivity that is about 0.1mm per metre. This should be good enough for setting up any machine likely to be found in a home workshop.

Levels are available with sensitivities up to ten times this that is 0.01mm/m.

For these levels to work accurately is is essential that they are “vertical” when being used. To achieve this as required they include a second level which is set so the top of the level is horizontal at right angles to its axis.

1074 an engineers level

1074 an engineers level

fig a Hahn and Kolb engineers level

Testing a level

A level can be tested by placing the level on a specific line on a flat surface. The flat surface is adjusted to be horizontal by the level. The level is turned round and set down along the same line. The level should still show that the surface is horizontal.

Use of levels

The main use of a level is when setting up a machine so a surface is level. For example, the bed of a lathe should be level along its length and across its width. Similarly a milling machine should be set up so the table is level along both the x and y directions.

Another use is that if one flat surface is level as is another flat surface then these two surfaces will be parallel. Consider the case of a tilting table mounted on a milling table. If the milling table is horizontal then one way of seeing if the top of the tilting table is horizontal is be means of a level.

Construction of levels

It might seem that a glass tube forced into a slight bend would make a level. However levels are not made like this. The curved is formed by grinding the inside surface of the glass tube.

see MEW

Measuring/setting levels over a long distance

Measuring whether a surface is level or not over a long distance appears to be a problem. Of course, if one can measure whether a surface is flat it is also possible to set a surface to be at a slight slope. This problem arises when building a long aqueduct as the Romans were keen on doing.

All the explanations I could find on how they did this were clearly written by people who had never really tried it. After some thought I came up with a solution  this is on page

Roman aqueducts

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