This lovely antique hour glass timer consisted of  two glass bulbs, joined in the middle by a collar made from fabric/wax- these are the earliest forms where the glass bulbs are separate. One of the glass bulbs is filled with sand or similar enclosed in a wooden cage. It would be turned upside for the sand to flow through to the glass bulb at the bottom. One of the spindles is an old replacement. These spindles were for protecting the precious glass bulbs behind.
This one  from the mid to late 1700s  takes approx an hour to run through. They were often used by the Royal Navy up until 1839  and for marines as a dependable measure of time whilst at sea.
It has also been documented that these timers were used in the church pulpit for measuring the parsons sermon to ensure it did not overrun what a good idea.!!!!! 
It measures approx 17.5cm in height x 9cm w.
These timers were made in various sizes for different requirements but the usual length of time was an hour, half hour or 3-4 minutes.