This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The spirit level consists of a glass tube partially filled with spirit, encased in a framework made of hard wood and protected by metallic facing on the most important sides. The quantity of spirit placed in the glass tube is just insufficient to fill it, so that a "bubble" of air perhaps 1/2 in. long always appears at the surface, being rendered visible by means of a sight-hole in the metallic plate which encloses and secures the glass tube in the wooden block. The ends of the glass tube are hermetically sealed when the proper quantity of spirit has been introduced. The wooden case or block must be perfectly level and true, and of a material that will not change its form by climatic or other influences. Average sizes are 8-14 in. in length and cost 2-10s.
Some are made with the sight-hole at the side instead of the top. Others have both top and side openings. Such is shown in Fig. 250, which represents Stanley's improved adjustable combined spirit and plumb level, by which it is possible to adjust a surface to a position both truly horizontal and truly perpendicular. The principle of action of the spirit level is that the air bubble contained in the glass tube will always travel towards the highest point; when it rests immediately in the centre of the sight-hole, a true level is obtained. It is necessary to remember, however, that it is only a guide to the level of that length of surface on which it lies; and in levelling longer surfaces the spirit level should be placed on a straight-edge instead of directly on the surface to be tested.