OMBROMETER. 17 



water of a neighboring river, the ordinary level of the surface- 

 water of a canal, the upper part of a wharf in mason-work, &c. 

 If the barometer has changed place, it is again necessary to 

 measure exactly its height above the same point of reference ; 

 the latter will serve to fix the height of the barometer and of the 

 station above the level of the ocean ; this datum being of the 

 greatest importance. Every change of this nature should be 

 carefully noted in the journal. 



It is greatly to be desired that the place of the barometer, 

 once determined, should not be changed, either from one story 

 to another, or from one house to another. If circumstances 

 compel this to be done, we should begin, before taking it from 

 its place, by raising the mercury in the cistern by means of the 

 screw, so as to fill the cistern and the tube ; it must then be 

 gently taken from the hook, turned upside down, and carried with 

 the cistern up, taking great care not to strike it against anything. 

 If it were transported without these precautions, even from one 

 chamber to another, great risk would infallibly be run of break- 

 ing it, or letting in air, and thus rendering it useless. 



Verification. From time to time the barometer should be so 

 inclined as to cause the mercury to strike gently against the top 

 of the tube. If it gives a dry and clear sound, it is free from 

 air, and the instrument is in good condition. If the sound is 

 flat and muffled, there is a little air in the barometric vacuum ; 

 and the fact should be noticed in the journal. Every occasion 

 should be seized to compare it anew with a standard barometer, 

 to ascertain whether it has undergone any change. 



OMBROMETER. 



Placing. The ombrometer, or rain-gage, is a funnel, accom- 

 panied by a graduated cylindrical glass vessel, and by a reservoir. 

 It should be placed in an open space. Trees, high buildings, 

 and other obstacles, if too near, may have a considerable influ- 

 ence in increasing or diminishing the quantity of rain which falls 

 into the funnel. The surface of the receiver should be placed 

 horizontally about six inches above the ground. The most sim- 

 ple mode of establishing it is the following : 



Place in the ground a cask or barrel (Fig. 9J, water-tight, the 



