690 MANUAL OF NATUKAL HISTOEY. 



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 



