SIGNAL SERVICE. 



arizing 

 while 



The adoption of this method of popul 

 und disseminating weather-knowledge, 

 the public. int.Tc-t in the data is fresh, will 

 vastly extend the benefits of the Service in all 

 -i'1-tions of the country, and familiarize the 



popular eye with the behavior and march of 

 atmospheric phenomena. 



To an untutored eye, it may seem trivial to 

 dwell upon apparently slight changes in baro- 

 metric and other conditions which are curso- 



WAR DEPARTMENT WEATHER MAP A DAILY METEOROLOGICAL CHART TRANSMITTED BY TELEGRAPH FROM WASH- 

 INGTON. 



rily glanced at on the weather-map ; but a mo- 

 ment's reflection shows the importance of ac- 

 curacy. A fall of only one tenth of an inch in 

 the barometer, trifling as it appears to be, indi- 

 cates the presence of an enormous force. The 

 normal atmospheric pressure, which is only 15 

 pounds upon a square inch, is 2,160 pounds 

 upon a square foot, and this amounts to about 

 30,000,000 tons upon every square mile. With 

 this pressure the column of mercury in the 

 barometric tube is SO'OO inches high. When, 

 therefore, it falls to 29'90 inches only, as any 

 one may calculate, over 100,000 tons of pres- 

 sure ara lifted up and removed from every square 

 mile over which the diminution of pressure ex- 

 tends. Conversely, if the barometer registers 

 on its scale so small an increase of pressure as 

 one tenth of an inch, it indicates the arrival 

 of a new mechanical power exerting an addi- 

 tional force of 100,000 tons weight to the square 

 mile. Such minuts but common barometric 

 changes, representing forces of great moment 

 in the oparation of the atmospheric machinery, 

 must not be overlooked in the deductions of 

 practical meteorology. But without the weath- 

 er-map of simultaneous observations the pres- 

 ence and influence of such changes can not be 

 detected and estimated. 



Signal Service Instruments. The necessity 

 for accurate observations in a system of weath- 

 er-telegraphy brings us to speak of the instru- 

 ments employed by the Signal Service Corps. 

 These have been selected from the best models 

 known, and subjected to experimental tests to 

 perfect their registrations. Every barometer, 

 thermometer, or other instrument nsed at the 

 stations undergoes thorough comparison with 

 the highest standards before it is sent out from 

 the office of the Chief Signal Officer, in which 

 there is a large apartment devoted to the work 

 of instrumental meteorologv, known as "the 

 instrument and model room." 



The barometer is the great dependence of tho 

 meteorologist, and upon its faithfnl accuracy in 

 n-iristering tho subtile yet momentous changes 

 of atmospheric pressure he must chiefly rely. 

 It measures tho weight, at tho spot where it is 

 lo-.-ated, of a column of air having a diameter 

 equal to that of its own tube. It has been in 

 use since the beginnin-jr of the eighteenth cen- 

 tury ; but not until ISM, when Adie construct- 

 ed a marine barometer, did it accomplish its 

 work satisfactorily. Fortin's barometer, how- 

 ever, became the most satisfactory for station*, 

 since it has the best cistern (having a flexible 

 base with a glass cylinder), gets rid of some 



