BOOK III. CHAR VII. 657 



Within, ■ 71 



July 23, Without doors, at nootr, 84 



Within, at I P.M. ^ 85 



On the i.ith of June the thermometer within doors, at nine in tlic 

 evening, was ^higher than at noon; thougla the thermometer out of 

 doors fell, ,cpnfiderably. The realon alligned for this was, that the- 

 air without was ftill warn-ier than the air within, and continued to 

 communicate its heat to tlic internal air. So Mr. Rouppe, at fea, in 

 the month of July,; 1760, in latitude 18. N., found the open air at 

 night fix degrees cooler than the air between decks. If t'lermometiersy 

 ufed in Jami'.ica, werefufpended in a ihady place out of doors, where 

 they would be more afFedled by changesin the atmofpliere, the tables 

 of heat and coolnefs for that ifland might be far more accurate. The 

 air within doors there, efpecially in the towns, is in general much 

 hotter than the air of any fliaded place without. Hence the degree 

 of heat raay be erroneoufly fuppojed many degrees higher than what 

 it really is. On the other hand, in England, where the houfes are 

 much loftier, tlie walls more mafly, and many apartments impervious 

 to the fun, and where its rays never enter into rooms of a Northern 

 expofure; the air within doors, on the liotteft days, is much cooler 

 than without ; and obfervations, taken there by a thermometer fuf- 

 pended always within doors, will confequently give a fallacious re- 

 gifter of the heat of fummer air, and often reprefent it to be much 

 lefs than it really is, as the example above cited moft clearly de- 

 monftrates. In making obfervations in Jamaica, (where the inha- 

 bitants are amphifcii) regard (hould be had to the fun's flation, whether 

 in the Northern or Southern Tropic. When he is in the former, 

 the inflrument fliould be placed in a room not inhabited, having a 

 South afpefl, the window left open ; and due care taken that the air 

 may freely, ehter, v/ithout any refiedted gleam of fun-fliine, or heat 

 from any wall or'building oppofite, 'Or too ftrong a light. When he 

 paffes to the Southern Tropic, it may be moved to the Northern lide 

 of the houfe, with the like precautious. 



The mercurial thermometers, 1 have reafon to believe, are the moll: 



to be depended on ; and they have this further advantage, that the 



ball is generally uncovered, fo that every variation of the atmofphere 



ipnmediately afFedts the whole mafs. Perhaps, two inllruments, or 



i Vol. III. 4 P even 



