Registering and Printing Barometer. 7 



carried by the float b, may always be maintained midway 

 between the two points p and p', and distant a little less 

 than the -^Vo of an inch from each. 



The barometer is of the siphon form ; the inside dia- 

 meter of the portions near the surface of the mercury is 

 nearly one inch. The upper and lower portions were 

 made from the same glass tube, the two being connected 

 by a tube of smaller diameter. We have not yet tested it 

 sufficiently to determine the correction for the difference 

 in size between the two legs. The observations, however, 

 so far seem to indicate a very perfect tube. 



The float b is of ivory ; the form, a paraboloid of revolu- 

 tion. The under side of this float is very slightly con- 

 cave. The diameter is one-eighth of an inch less than the 

 inside diameter of the tube, so that there is no friction be- 

 tween the sides of the float and glass. The platinum disk 

 d is supported by a steel wire passing through a brass 

 cap c fitted on the top of the tube, and an ivory disk 

 e inserted at a distance of 2J inches above the float b. 

 The ivory disk is connected with the brass cap by means 

 of two wires, so that it can readily be removed. A light 

 steel wire n passes through a hole in the cap, for the 

 purpose of preventing the disk d from revolving. This is 

 made sufficiently free to prevent any friction. 



The disk d is made of brass one-half an inch in dia- 

 meter, and is covered on both sides with platinum 

 plates. 



The platinum wire p is attached to a fine screw, for ad- 

 justing the distance of the points p and p' from the surface 

 of the disk d. 



These wires p and p r are, of course, insulated by being 

 attached to an ivory block, as shown in the figure. The 

 wires from these points are led to the top of the screw S, 

 where they are fastened to an ivory block, after which 

 they are connected with the electro-magnets m, m r . 



A fine platinum wire is inserted in the side of the 



