Your argument is invalid, sir!
As Keith from Bad Archaeology has very kindly linked to this blog in his latest post on dowsing (well worth a look by the way), I thought I’d post some period material gleaned in my recent trawling of the Scientific American archive that shows that whilst dowsing may be ancient, scepticism of it as a technique is by no means recent. The first is from 1856, and somewhat circumspect (though you can read between the lines):
‘Foreign Scientific Notes.
THE DIVINING ROD-The London Mining Journal states that the Rev. A Suckling, recently delivered a lecture at the St. Helliers, Jersey, on the history, antiquity, and correct principles of the ‘dowsing’ rod, for the discovery of minerals, metals, and springs of water below the surface of the earth. Mr. Suckling stated that he was convinced there existed a certain, though inexplicable, affinity between the effects of operations with the divining rod and what, in our present modern designation, is termed “mesmerism;” that he refers them to one and the same source. It was then attempted to be shown that mesmerism was known to the ancient Egyptians, and that many anecdotes and passages of Scripture show that it was well understood among the entire population of Asia. To this principle is ascribed the application of Naaman, captain of the host of Syria, to obtain a cure f or his leprosy, and the interview of Saul with the Witch of Endor. In the course of the lecture it was stated that many of the wells in the island had been discovered by himself and others, endowed with the peculiar power which was said to appertain only to certain persons.'
Just a year on however, and thinly-veiled eyebrow-raising is replaced by outright scepticism in this scathing comment;
‘TheDivining Rod a Deception.
The editor of the Saint Croix Union, published at Stillwater, Minn., says :- “The divining rod is an arrant humbug, and those using it, pretending that there is in the rod a mysterious and unaccountable virtue, are also humbugs. We know what we say, and intend it, too. Not only will a twig of a sweet apple tree point downwards in our hands, but a bifurcated twig of almost any tree will. We can take a twig of a willow, or an oak, or hickory, or anything, and hold it in our hands aud make it turn forty ways for Sunday. It isn’t a stream of water beneath us that does it, either, for we can make it point to a heap of ashes, or rock as hard as a nether millstone. It makes no difference. We don’t deny that water has been frequently found exactly beneath the spot indicated by the divining rod ; this has happened in our case more than once, but it is just as true also that, in numberless other cases that have come under our observation, men have dug long-dug deep-and spent stacks of money by digging where these aforesaid mysterious rods have pointed, and found no water.'
Although they haven’t quite put their fingers on the mechanism behind dowsing, others soon would, and by 1890s SciAm was recognising it in the oujia board;
G. A. S. says: I will be very glad to have you enlighten me as to the cause which makes the little table move and answer questions when using the game called “Ouija, or talking board.”
A. The hands. Hands off, no go.
You can almost hear the author saying ‘Next!’…
 Foreign Scientific Notes, Scientific American 11, 202-202 (8 March 1856) doi:10.1038/scientificamerican03081856-202
 The Divining Rod a Deception, Scientific American 12, 344-344 (4 July 1857) doi:10.1038/scientificamerican07041857-344a
 Notes and Queries, Scientific American 66, 74-75 (30 January 1892) doi:10.1038/scientificamerican01301892-74a