810 



SIGNAL SERVICE. 



errors, and is compactly put together. This 

 barometer, as afterward improved by the Kew 

 Observatory, and brought to the highest stan- 

 dard of effectiveness by Mr. James Green of 

 New York, is the one used by the Signal Office 

 at all its stations for obtaining the telegraphic 

 data. The instrument is kept in a room of as 



DIAGRAM OP PRESSURES IN A 8ECTTOK OP A CYCLONE. 



uniform a temperature as practicable, and in a 

 vertically suspended wooden box which can be 

 closed when the observer is not taking obser- 

 vations. For purposes of comparison and the 

 detection of any error, as well as to have a 

 substitute in case of accident, two barometers 

 are supplied to each station. Each instrument 

 after it comes from the maker's hands is sub- 

 jected to the Signal Office tests, and the resid- 

 ual errors are determined by comparison with 

 the great standard barometer kept at the office, 

 when a certificate of corrections is made out 

 and attached to the instrument ; it now becomes 

 a standard itself. Its readings may deviate to 

 a very slight extent from those of the " regula- 

 tor " ; but such deviations being known to a 

 thousandth part of an inch, allowance is made 

 for them whenever the observer makes his ba- 

 rometric report. As the elevation of the ba- 

 rometer above sea-level is determined for eacli 

 station, the proper correction for that is also 

 applied at each reading. 



Great care is taken in the location, correc- 

 tion, and reading of the Service thermometers, 

 which are of the highest standard. The instru- 

 ment is placed in the open air, so situated that it 

 will be always in the shade and yet have a free 

 circulation of air around, but exposed to no 

 currents of air, and beyond the influence of 



any artificial heat. Its surface is also carefully 

 protected and freed from rain or moisture of 

 any kind, and its bulb so placed as to have no 

 contact with the metallic scale or hack. Every 

 thermometer sent out to a signal-station under- 

 goes several previous crucial tests, and is 

 brought up to the standard kept in the instru- 

 ment-room of the central office, where 

 every error is corrected and recorded, and 

 the character of the instrument fully stu- 

 died. The maximum and minimum ther- 

 mometers are likewise tested, and the 

 slightest variations from the standard in- 

 struments determined by protracted ex- 

 perimentation, to the satisfaction of the 

 office, before they are issued to the ob- 

 servers. These instruments, by constant 

 and minute inspection of an officer detailed 

 to visit all the stations, as also by the rigid 

 scrutiny of the observers themselves, are 

 kept up to tlie highest point of accuracy 

 and precision. In the instrument-room of 

 the Washington office, 1,105 meteorological 

 instruments were last year carefully com- 

 pared with the '' official standards," and 

 982 were issued to the stations. 



The rain-gauge employed is also cor.- 

 structed with the utmost precision, to reg- 

 ister the amount of precipitation to inches 

 and tenths of an inch. This instrument 

 is placed with the top at least twelve 

 inches from the ground, and where it will 

 not be affected by local peculiarities or 

 obstructions from any object near by, so 

 that the rain as it descends from the clouds 

 may be fully caught and measured. It is 

 fixed firmly in a vertical position, and be- 

 yond the risk of being tampered with by un- 

 authorized hands. The rain-water collecting 

 in it is measured by a measuring-rod, graduated 

 to inches and tenths of inches ; snow is melted 

 and then measured in the same way. 



The wind-velocity measurer or anemometer, 

 which up to the present time has been found 

 the most satisfactory, is that of Eobinson. It 

 consists of four hemispherical cups revolving 

 in a horizontal plane and communicating their 

 motion to a vertical shaft or axis. In whatever 

 direction the wind blows, these cups will al- 

 ways be driven round with their convex sides 

 foremost, since the air presses with more effect 

 into the cups than on their exteriors. Experi- 

 ments have shown that the velocity of the cups 

 in all cases equals one third of that with which 

 the wind blows, no matter from what point of 

 the compass it comes ; and that this relation 

 between the velocity of the cups and that of 

 the winds is independent of the size of the in- 

 strument. By an arrangement of beveled wheels 

 every revolution of the cups is made, through 

 the shaft, to revolve a horizontal cylinder 

 carrying a pencil, which marks on prepared 

 paper the total number of revolutions made 

 by the cups. As the distance traveled by the 

 cups is three times that traveled by the wind, 

 the velocity of the latter can be easily deduced. 



