710 SEC. 14. METEOROLOGY. 



instrument consists of four parts, viz., the barometer, the thermometer for 

 measuring the temperature of the instruments, the thermometer for measuring 

 the temperature of the air, and the propelling and registering mechanism. 



The barometer is a balance-barometer of simple solid structure, with sup- 

 porting pillar and beam of iron. 



The thermometer for indicating the temperature of the instrument is a tube, 

 closed at the top and open below, filled with air, dipping into mercury, and 

 balanced by a weight on a string passing over a pulley. This tube moves 

 2 mm. for every degree C. of change in the temperature of the instrument. 



The other thermometer, intended for the measuring of the atmospheric 

 temperature, is likewise an air thermometer, consisting of a tube similar to 

 the above described, but which contains an iron tube, joined to a narrow lead 

 tube (2 to 3 mm. clear width), which again is connected with the thermo- 

 meter bulb. This bulb is freely exposed to the atmosphere, though protected 

 in a proper manner against rain and sunshine. It is made of copper, has a 

 capacity of 5 litres, and a corresponding surface area. Here, too, a change 

 of 1 C. gives a motion of 3 mm. 



Let b, t, T, signify pressure and temperature of the atmosphere and of the 

 instrument, x, y, z, the respective positions of the tubes I., II., and III. ; then 

 x = t\ (6, T), #=/ 2 (b, T), z=/ 3 (b, t, T), and hence b = $ l (.r, y), r = (j>. 2 (#,#), 

 t = <p (x, y, z), that is to say, the three quantities sought are determined by 

 the positions of the three tubes. 



The movements of the tubes are communicated to rods Avhich move verti- 

 cally in front of cylinders set revolving by means of a clockwork. This same 

 clockwork moves also hammers, which hit every 10 minutes upon the rods, 

 .whereby marks are produced upon the cylinders, and thus the positions of 

 the tubes noted. The clockwork is specially constructed for the instrument, 

 and the largest toothed wheel, with 120 teeth, has a diameter of 120 mm. 



Before each marking the tubes are slightly moved forward by steel cylin- 

 ders which dip into the mercury. All movements are produced by a hori- 

 zontal axis, which completes one revolution every 10 minutes, and which is 

 provided with the necessary wheels, &c. 



Electro-magnetism is used 



(1.) To mark the hours upon the cylinders. 



(2.) To work signals, which will indicate a possible stoppage of the clock- 

 work, or other interruptions in the working of the apparatus. 



(3.) To sound, by means of the clock and .a bell, the hours, thus giving 

 evidence of the right condition of all the agents employed. 



The advantages of the apparatus here described are : 



(1.) The constituent agents, the balance barometer, air thermometer, and 

 registering clockwork, are all well tested instruments. The movements to be 

 registered are so large that it is not necessary to magnify them. They can 

 be calculated beforehand by means of precise forniulse from constants that are 

 easily determined with great exactness. The apparatus has, therefore, nothing 

 of the nature of an interpolating instrument, a character belonging at present 

 to all registering mechanisms, but must be viewed as an instrument of precision. 

 Checking observations, daily required by all self-registering instruments, be- 

 come superfluous with this apparatus. 



(2.) The registering instruments can be put up in a dry place, protected 

 against changes of temperature. 



(3.) All parts of the apparatus are of metal and glass, wood is excluded, 

 The instrument is, therefore, more durable. 



(4.) The construction is extremely simple ; any disorder is easily repaired. 

 The purely mechanical motors are free from the disturbances to which 

 electro-magnetic motors are so frequently subject. If moderately well executed, 



