I am getting interested in barometers at the moment, as part of the research for a new book. It was a chance thing that began with finding 19th century references to leeches as indicators of the weather, and I'll come back to those later.

This morning, I chanced on this 1863 magazine article about James Joule:

A Mechanical Barometer.

An unique barometer for measuring small atmospheric disturbances, has recently been devised by Dr. Joule, of Manchester, England. It consists of a large glass carboy connected by a glass tube with a miniature gasometer, formed by inserting a small platinum crucible over a small vessel of water. The crucible is attached to the short end of a finely-suspended lever, multiplying its motion six times. When the apparatus was raised two feet the index moved through one inch; hence he was able in serene weather to observe the effect corresponding to the elevation of less than one inch.

The barometer is placed in a building, the slated roof of which affords, without perceptible draught, free communication with the external atmosphere. In this situation it was found that the slightest wind caused the index to oscillate, a gale occasioning oscillations of two inches, an increase of pressure being generally observed when the gusts took place. This barometer is undoubtedly very sensitive, and is highly spoken of amongst scientific circles in Manchester. It will however, of course, only show relative pressure, not absolute, as the indications would vary as much, or even more, by an Increase or diminution of temperature.

I'm still trying to puzzle out how that works, how it might be used, and what happened to it. That's my period, and I thought I knew just about all there was to be knowed about gadgets and such from the 19th century.

Still, the leeches were unknown to me as well.  They were written up in an 1854 article:

Leeches and their behaviour used to predict the weather:

A correspondent of the “Philadelphia North American” gives an interesting description of an ingenious instrument, contrived by Dr. Merryweather of Yorkshire, Eng., the great working principle of which is founded on the sensitiveness of leeches to the changes of the weather. It is well known that leeches confined in a bottle partly filled with water, are accustomed, previous to a storm, to rouse from their sluggishness and exhibit signs of extraordinary perturbation. They will swim in all directions, and rising one after another to the top of the water, commence climbing the side of the bottle."

Availing himself of this time-honored custom among leeches, Dr. Merryweather arranged a number of bottles on a stand, each containing a leech and a metallic tube of a particular form, covered with shellac varnish, so that no metal could come in contact with the animal.— When a change in the weather was about to take place, the leeches would crawl into this metallic tube, and in so doing displace a small piece of whalebone which was arranged so as to partially close the opening. To this whalebone was attached a wire, which, passing upward through the mouth of the bottle, connected with the hammer of a bell, so that whenever the leeches were influenced by the electro-magnetic state of the atmosphere to ascend the tube, notice of the fact would be promptly transmitted to the ears of their master.

But it is not absolutely necessary that every one should have such a finished apparatus as that of Dr. Merryweather. On board of vessels it would only be necessary to keep a few leeches in a bottle, placed in some prominent place where the lookout could occasionally examine their movements, and the necessary warning be conveyed in ample time.

Dr. Merryweather seems to have tested his invention fairly. For an entire year (1850) he wrote to the president of the Philosophical Society of Whitby, accounts of the storm indications of his leeches; and in no instance did they prove incorrect. If these results are verified by other observations, a leech barometer may be deemed an indispensable appendage to every ship and every household.

All of these and many other gems can be found in the online Cornell University archive of Scientific American, covering 1846 to 1869, located at http://cdl.library.cornell.edu/moa/browse.journals/scia.html — this link lets you search, browse or poke around. It's a rich interface, and it repays some fiddling time to get to used to all of the bells and whistles.

And now, back to curious 19th century cures for cholera. After that, back to the leeches, and the 19th century Australian export trade in leeches to Europe.  As early as the 1840s, W. A. leeches were being exported to Mauritius, but I don't know why.  Not yet, anyhow.