102 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



equilibrium by a small spring, the amount of change in the 

 weight of the mass, either from rain-fall or evaporation, be- 

 ing; indicated on the scales of a delicate balance. In order to 

 secure the mechanical record of the hourly variations in the 

 weight of the vessel and of its contents, the professor causes 

 the lever to vibrate between two platinum points so placed 

 that whenever a change in the weight of the vessel by a 

 given amount (say ten grains) takes place a magnetic circuit 

 will be established, passing through an electro-magnet. A 

 micrometer screw will then be operated by means of clock- 

 work, thereby tracing a curve on a revolving drum, precisely 

 as in the case of the self-recording barometer and thermom- 

 eter. Proc. Amer. Assoc, 1873. 



joule's kew barometer. 



Dr. Joule, of Manchester, describes in the proceedings of 

 the Literary Physical Society of that city the method of con- 

 struction of a new barometer. The tube of this instrument 

 has a bore of about three sixteenths of an inch, is bent into a 

 siphon shape, and near the longer end is drawn to a capillary 

 tube. After having been thoroughly cleaned with nitric and 

 sulphuric acids, the short end of the tube is filled with mer- 

 cury, the end of the longer limb being attached to the mer- 

 curial exhauster, an instrument apparently on the principle 

 of the Sprengel air-pump. On working the exhauster the 

 mercury rises in the long tube of the barometer, and being 

 replenished by pouring more into the short limb, the longer 

 column soon rises to the height due to the atmospheric press- 

 ure. The acid left adhering to the inside of the tube accumu- 

 lates on top of the longer column of mercury to the depth of 

 one third or one half of an inch. All bubbles of air or other 

 gases disappear in the course of two or three days. The cap- 

 illary part of the tube is at length sealed, and the barom- 

 eter is complete, a small quantity of sulphuric acid being 

 added to the shorter end of the tube. Mr. Joule states that 

 the instrument thus formed has the utmost facility in its 

 movement, so that the most minute changes of pressure are 

 registered without any dragging. The depression produced 

 by capillary action is reduced by one half. 



