Hold the instrument vertically and look through the line of sight and horizon glass.
"Lectures in Navigation"
Ernest Gallaudet Draper
His teeth, too, are more vertically inserted, and the nasal bones less depressed.
"The Ethnology of the British Colonies and Dependencies"
Robert Gordon Latham
If we take two equal lengths of wire, bent at right angles, and connect the point of intersection with the earth, placing one of these wires vertically and the other horizontally, we then have an arrangement which responds to the impact of electric waves, and has electrical oscillations set up in it in such fashion that the common point of the two wires has a very small amplitude of potential, but the two extremities have equal and large variations.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming