1848.] Observations made on a Botanical Excursion. 357 



A thermometer with the bulb blackened affords the only means the 

 traveller can generally compass, if measuring the power of the sun's 

 rays. It will be seen that by this I have recorded a greater amount 

 of solar heat than was supposed usual in India. 



A good Photometer being still a desideratum, I had recourse to the 

 old wedge of colored glass : — that used was so constructed as to be 

 equivalent to a wedge of a uniform neutral tint, the distance between 

 whose extremes, or between perfect transparency and total opacity was 

 equal to 12 inches. A moveable arm carrying a brass plate with a slit 

 and a vernier, enables the observer to read off at the vanishing point of 

 the sun's limb, to y^o tn of an inch. I generally took the mean of four 

 or five observations, but place little dependence upon the results. The 

 causes of error are too obvious for notice here. As far as the effects of 

 the sun's light on vegetation are concerned, lam inclined to think that 

 it is of more importance to register the number of hours or rather of 

 parts of each hour, that the sua shines, and its clearness, during the 

 time. To secure valuable results this should be done repeatedly, and 

 the strength of the rays by the black bulb thermometer registered 

 at each hour. 



Finally, with regard to the hours at which the observations were 

 taken, the three principal ones, 9 a. m., 3 p. m. and 9p.m. were those 

 adopted by the antarctic expedition. A morning observation was added, 

 because the 3 a. m. one is seldom available for the traveller especially if, 

 besides the toils of the march he has other pursuits. The most useful 

 observations at that hour are perhaps those for the temperature of the 

 grass, soil, &c, which vary little for many consecutive hours in the night, 

 and are losing by radiation till the sun's power is felt. 



I much regret not being at present able to enter into these computa- 

 tions, which would render the following observations more useful. I 

 have preferred recording them thus early to detaining them for an indefi- 

 nite period. Their publication will enable many to point out to me 

 better modes of observation ; and direct a few how to conduct such 

 enquiries. I would also hope there are some who are, like myself, 

 seeking for comparative observations, and to whom these will be wel- 

 come, as are all similar ones, made in other parts of India, to me. 



The more important results which these will give, with more or 

 less accuracy are : — 



3 b 2 



