262 CLIMATOLOGY. 



In the observatory the mean for the same period was 

 only — I5'9l° Fah., and the range no more than 0-97°. The 

 maximum occurred at 6 P.M., being retarded 4f hours; 

 and the minimum at 10 a.m., being delayed 3 hours. For 

 most of the night the temperature was above the mean, — 

 such being the effect of the interposition of the building 

 between the thermometer and the blue atmosphere. The 

 walls of the observatory, it is necessary to remark, were by 

 no means air-tight, and the door was opened at least once 

 an hour in the day, and sometimes, especially on term days, 

 much oftener. There was, consequently, a considerable and 

 frequent admission of the external air ; and, on the other 

 hand, during the experiments on magnetic intensity, the 

 heat of the observer's body had an evident effect in raising 

 the temperature of the room. 



I had intended to have instituted a series of observa- 

 tions, with Sir John Herschell's actinometer, on the noc- 

 turnal radiation, and also on the momentary intensity of 

 the direct rays of the sun ; but the instrument was un- 

 fortunately broken on the voyage. The Edinburgh New 

 Philosophical Journal for 1841 contains the results of 

 observations made at Fort Franklin, with the black bulb 

 thermometer, on the heating poAver of the sun's rays, and 

 I renewed these observations at Fort Confidence ; but, 

 as they were not carried on later than April, they furnish 

 no information respecting the power of the sun in the 

 months in which the px'ocesses of vegetation are active. 

 As the black bulb thermometer indicates the accumulative 

 effect of the sun's rays, it seems to be an useful instru- 

 ment for ascertaining the heating power of the sun on the 

 stems and larger branches of trees, at least, if not also on 

 their leaves and on herbaceous plants. The hybernation 

 of trees ceases long before the temperature of the at- 

 mosphere is sufficient to restore activity to the vegetative 



