Chap. 8.] Reafon of Autumnal Colds, &c. 411 



plain, whence it is that fleeping in damp beds and in- 

 habiting damp houfes is fo very dangerous, and why 

 the evening air isfo pernicious in fummer and autumn, 

 and why it is not fo in the hard frofts of winter. It 

 has puzzled many to account for the manner in which 

 fuch an extraordinary degree, or rather quantity of 

 heat is generated, which an animal body is fuppofed to 

 lofe if expofed to the cold of winter, which it com- 

 municates to the furrounding atmofphere in warm 

 fummer weather; but is it not more than probable, 

 that the difference of the quantities of heat actually 

 loft or confumed, is infinitely lefs than what they 

 have imagined*?' 



Various inflruments have been invented under the 

 general name of hygrometers f, for afcertaining the 

 quantity of moifture contained in the atmofphere. 

 Mod bodies attract moifture, and are expanded by it. 

 Wood and other folid bodies are fwelled by the moif- 

 ture infmuating itfelf between the fibres, and confe- 

 quently a piece of wood cut tranfverfely, will be ex- 

 tended in length by the abforption of damp or wet. 

 Cord, catgut, &c. the fibres of which extend longitu- 

 dinally, will increafe in thicknefs, but will contract in 

 length on the application of moifture. On this laft 

 principle the common weather-houfe is conftructed, 

 which is no bad hygrometer for general purpofes ; the 

 contraction of the itring by wet forces the man out of 

 the door, and when by the return of fine weather the 

 ftring or catgut is difpofed to re fume its natural length, 

 an elaftic wire acts upon it, and the worn. in appears. 

 The firft regular and graduated hygrometer that de- 

 ferves to be mentioned, as made in this country, was 



* Thompfon's Experiments. Phil. Tranf. Vol. Ixxvi. 

 f 'Yyoj, (hygros) "moifture;" and Mfyor, (metron) "amea- 

 frre." 



that 



