166 M, Ramond*s Instructions for the Application of [Oct. 



^nciicated by the frequent variations of the barometer and ther- 

 mometer. 



8. To prefer times when the barometer is near its mean 

 height, rather than its extremes. 



9. To give continual attention to the variations of the ther- 

 mometers. The mistakes made in estimating tlie real tempera- 

 ture of the mercury and the air are the origin of the most consi- 

 derable and the most common errors. 



10. To pay not less constant attention, both to the disposi- 

 tions of the atmosphere, and to local influences which may affect 

 the accuracy of the measurements. To take exact notice of the 

 direction of the winds, the movements of the clouds, the pre- 

 sence or absence of the sun, and to observe the variations of the 

 instruments in relation to these circumstances. 



11. To be doubtful of operations made in very changeable 

 weather, and especially if the air is not uniformly modified at 

 the two stations, as happens when different winds prevail at 

 each ; when one enjoys the sun, while the other is clouded, or 

 encompassed with mists ; when the decrease of temperature is 

 nothing, or inverted, &c. 



If the constitution of the day is such as to be rr^markable by 

 any thing excessive, either in the temperature, or in the eleva- 

 tion or depression of the barometer, to repeat the operation in 

 ordinary weather in order to verify the former result ; or, in cir- 

 cumstances directly opposite, to correct by compensation 

 contrary errors. 



12. If the horizontal distance be very great, to repeat the 

 operation several times. If it be excessive, to rely only on 

 means deduced from a great number of observations always 

 simultaneous. Less than a year will not be sufficient to deter- 

 mine small differences of elevation between places very distant ; 

 and if the distance be such that the climates of the respective 

 places should be sensibly different, no barometric mean will 

 determine exactly their relative elevation. 



13. To conform to these rules : and in so doing, to use in 

 the observations precision and dexterity ; in the examination of 

 all circumstances, to take a just view, and use sound discrimi- 

 nation ; and then I can venture to answer that the observer will 

 not be deceived either by the instrument, or the formula. 



If circumstances should positively require the sacrifice of any 

 of the prescribed conditions, we shall judge of the merits of the 

 operation by the value of the condition sacrificed. 



Btit it may be asked, shall v/e content ourselves with mere 

 approximate measurements ? Then only, we may reply when we 

 observe as well as circumstances permit. Approximate measure- 

 ments are not to be disregarded when we only take them as 

 such ; and when we have not the means of procuring better. It is 

 still a great instance of utility in the barometer to teach us in an in- 

 stant, and without difficulty, that which with much apparatus and 

 loss of time, other instruments will not often teach us equally well. 



