aflfected by the radiation, reflection or conduction of heat front 



neighboring bodies, or by its observation at such hours as will 

 not represent the mean of the twenty-four ; by the deteriora- 

 tion, of the barometer from the intrusion of air into the tube, 

 by its wanting an apparatus for ascertaining its zero, and by 

 its being observed at hours which will not exhibit either the 

 daily oscillations of the column, or its mean annual height. 

 A remarkable instance of labor almost thrown away, is found 

 in a series of observations made by a Professor in a celebrated 

 college, under the direction of a learned Society, where the 

 times of observation of the barometer were those, neither of its 



of them was the fluctuating period of sunrise, and where, from 

 the close agreement of the results, he pronounced the notion 

 of a daily oscillation of the column, not to be supported by 

 these observations. In this case the observations were nearly 

 valueless, from the selection of improper hours, and the deduc- 

 tion of the inference showed a want of caution truly surpris- 

 ing. Yet even with these drawbacks, these observations are 

 better than none at all, although a Uttle attention to the sub- 

 ject would have rendered them almost inestimable. While, 

 however, care and accuracy are to be always studied, I would 

 warn you against the opposite fault, of undervaluing your own 

 observations because they may not possess the latter quahty in 

 the highest possible degree ; this feeling when carried to its 



ly detrimental to the cause of science, but also, and in a much 

 higher degree, to the individual himself. Some confidence in 

 ourselves is requisite here as well as in every occupation of life; 

 and he who fears to relate what he has observed, lest he may 

 be repeating a twice told tale, will never do much to advance 

 science or his reputation. Guarding, then, by all the means 

 \\nth which we may be acquainted, against inaccuracies, I 

 would impress on each of you, if possible, the conviction of the 

 importance of forming the habit of observing and recording 

 some natural phenomena at stated periods of each day, the 

 rapidity with which it will become a source of pleasure, will 

 soon add an agreeable surprise. Take, for instance, the direc- 



