INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 17 



proach each other slightly, returning again or separating still farther 

 with diminution of pressure. To measure the changes in atmosphere 

 pressure, it is only necessary to measure the minute movements of 

 this flexible cell. 



In the common aneroid a lever, Z, attached directly to the spring, 

 connects by a link, m, with a very short arm of a sort of bell-crank 

 lever, r, t^ having a horizontal axis on pivots at each end. 



The longer arm, t^ of this bell-crank lever is connected by means 

 of a wire, s^ with a very fine chain, the other end of which winds 

 around a small wheel or drum on the axis, a, upon which is mounted 

 the hand as seen. At 5 is shown a small spiral steel spring, like the 

 hairspring of a watch, which serves to take up the slack in the loose 

 connections of the numerous joints, levers, and links. 



At r is shown, also, a small counterpoise weight attached to the 

 bell-crank lever to aid in securing a more stable position of the index 

 when the barometer is placed in different positions ; that is, whether 

 the dial is horizontal, or vertical, or turned to one side or the other. 



The point of attachment of the link, m^ to the bell-crank lever is 

 sometimes adjustable so that the movements of the hand can be made 

 to correspond to the value of the scale graduations. 



The steel spring, B^ is also slightly adjustable by means of a screw 

 from the underside threaded into the part, N. This permits adjust- 

 ing the hand to any particular point of the scale to give correct 

 readings. 



Effects of temperature. — The steel spring and the feebler elastic 

 reaction of the composition metal of the vacuum chamber are ap- 

 preciably weakened by increase of temperature, so that in some 

 cases a rise of the pressure may seem to occur which is really caused 

 by the weakening of the spring. In some cases efforts are made to 

 compensate for this by leaving a small quantity of air in the vacuum 

 chamber, which when heated increases its pressure upward and tends 

 to offset the weakening effect upon the springs. A better plan is to 

 make the lever, ?, of two different metals, viz, brass and iron, firmly 

 brazed together. The differential expansion of these two metals with 

 temperature changes produces flexure in the lever. By filing and 

 adjusting the bimetallic bar the flexure due to temperature can be 

 made very nearly to balance the effect of temperature on the spring. 

 The aneroid is then said to be "compensated" and this word is often 

 found on the dial. In many cases this word is there when the? 

 compensation is very imperfect. 



Aneroid harographs. — Extremely simple and portable barographs 

 are constructed upon the aneroid principle, of which that of Richard, 

 being widely used, is here described (fig. 9). 



It consists of a cylinder, yl, on which the recording paper is wound, 

 revolving once a week by means of a clockwork contained inside. 

 A series of corrugated metallic shells, 5, eight in number, joined one 

 above the other and exhausted of air, forms an aneroid system eight 

 times as sensitive as a single chamber. The movement of the shells 

 is still further greatly magnified and is transmitted to the recording 

 pen, G^ by a series of connecting levers. The pen may be released 

 from contact with the paper by pushing the lever, Z>, to the right. 



The corrugated shells are the same as used in ordinary aneroids, 

 the steel springs for distending the shells being placed inside. The 



