TERRESTRIAL REFRACTIOX. 35 



and if we take those by niebt, it will be ?•=— — of 

 the contained arc, from which we can collect no- 

 thin! 



^o* 



4th. I have now only to add a few words on the 

 comparative density of the air, at the different times of 

 observation, such as entered in the tables ; and the 

 evident want of connection, between its changes and 

 those in the refraction; from which we may infer, that, 

 although in northern climates (where the mercury 

 will rise and fall several inches in one day) observa- 

 tions of the barometer and thermometer may be at- 

 tended to with advantage, on the contrary, in tropi- 

 cal countries, where (as appears in the present expe- 

 riments) the variations of the mercury are hardly dis- 

 cernible*, those instruments will prove perfectly in- 

 efficient. 



APPENDIX. 



An accoimt of Experiments on the fibres or beards of 

 the Panimooloo grass ; con tabling also an account of 

 the construction of the hygromettrSj used in the pre- 

 ceding Ejcperiments. 



Before I proceed to give an account of the experi- 

 ments, by means of which I satisfied myself of the 

 conipetency of the beards of the Panimooloo grass, to 

 the object I had in view, it will be necesary to give 

 a short description of the plant itself, or (since a bo- 

 tanical account of it is not here intended) of that 

 part of the plant, which was used in the construction 



* At Madras, the greatest alteration was ob?(ii:ve(i 0.73 inches: 

 and in the mean anuual change 0. 53. At Bansii\iry, during the 

 time of attending to the preser.t experiments (that is about twelve 

 days) tiie greatest change was 0. 2 ; the iatter place being 2970. $ 

 feet above the leyel of the sea. 



