THE SMITHSONIAN INSTITUTION. 29 



On account of the delicate and peculiar nature of the apparatus employed, the 

 Magnetic Observatory is not accessible to the public. 



Observations are made at 7 A. M., 2 and 9 P. M. every day, of the barometer, 

 thermometer, psychrometer, the direction of the wind, clouds, amount of rain, etc. 

 These observations are carefully computed, together with those received from the 

 Smithsonian corps of observers in every part of the country ; and the material 

 is thus accumulating for a valuable work on the meteorology of the United States. 

 Blanks, instructions, and tables are furnished gratuitously to persons who will 

 make observations. Instruments are supplied when requested, but at the expense 

 of the parties ordering — the income of the Institution being, as yet, insufficient 

 to meet such and other like desirable outlays. 



THE GREAT BAROMETER. 



The instrument noticed in the hall, near the entrance, is a Sulphuric Acid Bar- 

 ometer, constructed by James Green, 422 Broadway, N. Y., expressly for the In- 

 stitution. 



The glass tube is 240 inches long, and f ths of an inch in diametei^ and is enclosed 

 in a cylindrical brass case of the same length, and 2 J inches diameter. The glass 

 tube is secured in the axis of the brass case by a number of cork collars, placed 

 at intervals, which while they prevent all lateral displacement of the tube, enable 

 it to be moved upwards and downwards for the adjustment of the zero-point. 



The reservoir consists of a cylindrical glass bottle of four inches in diameter, 

 with two openings at the top; one in the axis to admit the lower end of the long 

 tube, which is tapered to about one-half of the general diameter, the other to 

 transmit the varying pressure of the atmosphere. 



The scale for reading the elevation is divided into inches and tenths, and by 

 means of a vernier, moved by a rack and pinion, the variations can be measured 

 to a hundredth of an inch, and estimated to a still smaller division. 



The drying apparatus, placed between the external air and the interior of the 

 reservoir, consists of a tubulated bottle with two openings, containing chloride of 

 calcium, and connected with the reservoir by an india-rubber tube, by which ar- 

 rangement the air is deprived of its moisture. 



To ascertain the temperature of the column of the liquid, two thermometers are 

 attached, one at the top and the other near the bottom. 



The advantages of the use of sulphuric acid are 



1st. That it gives off no appreciable vapor at any atmospheric temperature ; 

 and 2nd. That it does not absorb or transmit air. 



A full account of this instrument is given in the proceedings of the American 

 Association, for the advancement of science, published by Jos. Levering, Cam- 

 bridge, Mass., 1857, p. 135. 



On the top of the high tower of the Smithsonian building, and also on the 

 grounds connected with it, may be seen a number of eain gages. 



Several forms of this instrument have been used, but the one which has been 

 found the best, under all circumstances, is shown in the annexed fi gure. 



