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.
LSD is rapidly absorbed after an oral dose and initial symptoms usually appear within a few minutes; visual illusions and perceptual distortions occur within the second or third hour (3). First perception of drug action occurred as late as 2-4 hours after oral doses of DOB in 4 of 13 subjects (2). A pharmacokinetic study (4) showed that DOB (or a metabolite) began to concentrate in the brain only 3 hours after an oral dose. DOB is said (1) to have a longer duration, 12-24 hours, than LSD, 8-12 hours. Both of these characteristics of DOB match those of the drug ingested in Dr. Lake and associates' case --slow onset (3 hours) and a long recovery time (18-24 hours)- as well as or better than do those of LSD.