New York Weather Bureau, 417 



1. The standard, a Fuess normal siphon baromeiter_, is so con- 

 structed that any error due to the presence of air at the top of the 

 mercurial column may be detected and approximately corrected 

 by varying the capacity of the cistern, and so making the column 

 occupy more or less of the vacuum chamber at the top. The press- 

 ure of any gas in the vacuum chamber will increase as the space 

 is diminished, so that, under these conditions^ the readings will 

 not agree. 



2. A Draper barograph furnishes a continuous record of press- 

 ure. The cistern of this instrument is suspended upon long spiral 

 springs, while the main tube is fixed. The proportions of the va- 

 rious parts are such that a rise of pressure forces the mercury up 

 into the tube from the cistern, which then rises; and a decreasing 

 pressure produces a contrary effect. The record is made by a pen 

 attached to the cistern, which bears against a record sheet moved 

 horizontally by clock work. Since this barograph acts by balanc- 

 ing the weight of the mercurial column against the weight of the 

 atmosphere, the correction for the temperature of the mercurial 

 column is very small, falling within the limits of other errors of 

 the instrument. The latter seldom amount to 0.01 inch, 



3. A Green's Fortin barometer of the ordinary pattern, having 

 a fixed zero point, to which the level of the cistern is adjusted^ is 

 used for purposes of current comparisons. 



Thermometers. — 1, Standard dry bulb and maximum and mini- 

 mum thermometers are mounted in a shelter of the Weather Bu- 

 reau pattern. The shelter stands about 40 feet east of the college 

 building, upon a grass plot sloping downward toward the west; 

 the height of the instruments above the sod being about 6 feet. 



2. A continuous record of temperature is furnished by a Richard 

 thermograph placed in the shelter. The pen tracing the record 

 is attached to an arm whose position is governed by the expan- 

 sion or contraction of a metal tube filled with alcohol. The record- 

 ing sheet is carried by a cylinder, revolved by clock-work, at the 

 rate of one turn per week. This thermograph, which is very accu- 

 rate when properly adjusted, is checked by comparison with the 

 adjacent standard thermometer. 

 27 



