424 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



index to show the motion of the varying' changes in the weight of tlio 

 atmosphere. But this instrument was fonmd to be useless on account of 

 the expansion of the metallic case, caused by the variation in tlie tempe- 

 rature at different iieights from tlie surface of the earth. In 1845 or '46 

 M. Vidi made an improvement on this style; he made a flat circular box, 

 corrugated top and bottom and soldered to a rim, the corrugation was to 

 give the flat sides greater elasticity. It was said that Vidi used a com- 

 pensating case inside the corrugated disks, but the difference between his 

 and Conte's is simply this, that Vidi leaves sufficient air inside the plates 

 to correspond to a pressure of half an inch of the mercurial barometer, so 

 that the expansion and contracticm of the enclosed air serves to correct the 

 opposite contraction and expansion of the case, and in this consists the 

 whole of M. Vidft's secret. If a perfect vacuum is made between the plates 

 the index goes up by heat, and if there .is too much air left in, it will also 

 go up by an increase of temperature. So I found in my first experiments 

 tliat a perfect vacuum would not answer, and I now make a vacuum in my 

 barometers on a clear dry day. If the small quajitity of air enclosed is 

 damp it has a very bad effect, so I keep them in a dry room for twenty-four 

 hours, and then they are put into a box with quick lime and left until' the 

 weather is very dry, or the barometer is very high, they are then gauged . 

 so that when the mercurial barometer marks twenty-nine, the aneroid marks 

 twenty-eight and one half inches; there is always a difference of half an 

 inch between the two barometers while they are being graduated. During 

 my experiments I found that metals were weakened by cold and strength- 

 ened by heat, and that a strip of steel or otlier metal has a varying strength 

 according to the temperature in which it is placed. At first I thought this 

 was due to dilatation, but I found that this was not the cause. If we take 

 a solid block of metal and heat it, we will find it to expand a given bulk; 

 now if we make a hollow box of the same size as the solid one, and heat 

 it to the same degree, we will find it expand to precisely the same bulk; so 

 that whether hollow or solid it will increase the same in size for an equal 

 temperature. 



The machinery of the Aneroid Barometer that gives motion to the index 

 . is very simple, and very slight changes are indicated by it. The barome- 

 ters are made of different sizes; the largest I make are ten inches in 

 diameter, the cheapest are sold for S6.50. 



The Chairman. — A very large barometer was made at the suggestion of 

 Prof. Ellet a few years ago, under the direction of Prof. Henry, for the 

 Smithsonian Institute, It was filled with sulphuric acid of a specific grav- 

 ity of 1.8, that is, nearly twice as heavy as water. The tube must, there- 

 fore, be about fifteen feet long. The object was to get a very large range 

 of measurement. Sulphuric acid has an intense attraction for water, this 

 was, doubtless, guarded against in that instrument, which I have never 

 had the pleasure of seeing, 



Mr. Tagliabue. — Tliat barometer was made by Mr. Green of this city, a 

 well known mathematical instrument maker. 



Dr. Warren Howell. — Captains going to sea usually take both the aneroid 

 and mercurial barometers with them. They consider the arenoid the most 



