The instrument for measuring atmospheric pressure, the barometer, is calibrated to read zero when there is a complete vacuum; the pressure indicated by the instrument is therefore called absolute pressure. The term pressure gauge is commonly applied to the other instruments used for measuring pressure.
They see what's already there in a different way, and most of the time, they're aware that their altered perceptions are caused by the drug. After taking LSD, the effects -- known as a trip -- usually start within an hour and can last up to 12 hours, with a peak about halfway through the experience.
The effects of LSD can vary greatly, depending on factors such as previous experiences, state of mind and environment, as well as dose strength. Generally, LSD causes expansion and altered experience of senses, emotions, memories, and awareness for 8 to 14 hours. In addition, LSD usually produces visual effects such as moving geometric patterns, trails behind moving objects, and brilliant colors.