174 ^' ttamqjid^s Instruct ions for the Application of [Sept. 



of which is too small either to communicate its heat to it, or to 

 create an obstacle to the free circulation of the air, when the 

 thermometer is fixed at a due distance from it. I have always 

 followed the sauie plan. A staff of about six feet in length 

 jinswers the purpose ; it has an iron point to fix it in the ground. 

 At the other end are two holes to receive two small arms of iron 

 or brass, about five inches long ; one terminated by a hook, the 

 other by a ring, the end of each which is to fit into the hole is 

 formed into ^. screw. Th^ arm with the hook is fixed in the 

 upper hole, and serves to suspend the thermometer ; that with 

 the ring in the lower, and retains it in a position parallel with 

 that of the staff. The staff serves for myself or my guide, and 

 the arms are carried in the bag which holds the thermometers. 

 I do not believe it possible to attain the requisite precision in the 

 ii)4ici^tio'^s with less apparatus. 



The observation of the thermometers is the most delicate and 

 the most difficult part of the operations ; and most of the faults 

 which we commit in the measurement of heights may be traced 

 to a false valuation of the temperature of the air, or of that of 

 mercury. I have mentioned this before, but there is no harm in 

 repeating it, and we cannot be too careful in pointing out the 

 sources of error, especially when they are of such a nature as 

 easily to disguise themselves to inattentive eyes. The inexpe- 

 rienced observer, when he meets with unsatisfactory results, 

 will be less tempted to lay the blame on his instruments, or on 

 his forjpula ; and this may often spare men of speculative minds 

 the trouble of imagining new theories to correct in the formula, 

 ifregularities, which exist only in the observations. 



All philosophers who have been engaged in barometrical 

 measurements must doubtless have made the same remark which 

 I have. They cannot have carried the thermometer to summits 

 of mountains, exposed to all winds, without often experiencing 

 the same embarrassment which I have. The thermometer has 

 varied, with them as well as with me, every instant in proportion 

 to the degree of wind, of calm, the presence of the sun, or the 

 interposition of clouds. These variations they have not neg- 

 lected, because they could not misinterpret them. Like me they 

 l]ave often \\^en in uncertainty as to the real and intrinsic tem- 

 perature of the air, and that of their instruments ; and have not 

 assuredly confined themselves to noting down at all hazard 

 merely the temperature which any accident may cause to prevail 

 at the exact moment of observation. But if they are generally 

 silent on a point which caniiot escape any experienced person, 

 it is that they suppose the logic of the observations familiar to all 

 those who employ meteorological instruments with any discern- 

 ment. I conceive I ought not to imitate them in addressing 

 myself to beginners. Such readers require advice and exam- 

 ples. I will give one or two, and I do not select the most rare 

 cases. 



