Mars is notoriously hard to observe from ground-based observatories. The proposed canals were 'observed' when there was no photography and were at the limit of the resolution of the telescope. They were doubted by the scientific community at the time, but the idea was picked up by the press and became a popular sensation.<quoted text>
And scientists thought there were canals on Mars.
Lesson: don't get your science from a sensational popular press.