690 MANUAL OF NATUBAL HISTOKY, 



well as all those appearances in the heavens, of more 

 or less frequent occurrence, such as the rainbow, 

 waterspout, halos, mock-suns by day, or the fleeting 

 aurora by night. 



The instruments and apparatus which will be 

 requisite for such enquiries, are, — 



1. A Barometer. — A portable one is the most 

 generally useful, but where it can be managed it is 

 advisable to carry also a standard one for comparison. 

 The lately invented "Aneroid Barometer" is, for 

 many purposes, very appropriate ; it bears carriage 

 well, and is not injured by being shaken ; it is also 

 extremely sensitive, more so than the mercurial in- 

 strument ; we have ourselves frequently watched 

 the index falling as a squall or heavy cloud was 

 passing overhead, and rising immediately after- 

 wards, indeed a good one will distinctly indicate the 

 difference in the atmospherical pressure for every 

 altitude of eight or ten feet ; we have seen the height 

 of mountains varying from 3000 to 4000 feet, ascer- 

 tained by its means, and its correctness afterwards 

 checked by trigonometrical measurement. 



2. Thermometers. — Of these, several should be 

 provided. One, well tested and minutely graduated, 

 should be carefully kept for comparison; metallic 

 frames are preferable to wooden ones, as the latter 

 warp. One thermometer, for ascertaining the tem- 

 perature of fluids, should have the bulb projecting 

 an inch and-a-half beyond the foot of the scale, and 

 be carefully packed up in a soft padded case. For 

 very cold climates alcohol must be used instead of 



