50 



Abstract of Eight Months' Meteorological 



[July 



Professor Forbes* from a comparison of most of the recorded ob- 

 servations, has deduced a formula for finding the mean oscillation be- 

 tween 10 a. m. and 4 p. m. at any given latitude. The result of this for 

 latitude 16° 30' is 2.35 millimetres or 0.092 of an inch. The mean of 

 the observations in the table cannot be considered the experimental 

 mean at Moulmein, for, three out of the four unrecorded months being 

 rainy, the former would almost certainly exceed the latter. The mean 

 of 0.133 the greatest and of 0,077 the least quantities in the table, 

 equal to 0.105, is probably very near the true mean. The difference 

 between this and the result of the formula is a less fraction of the 

 whole oscillation than occurs in a similar comparison in lat. 56° be- 

 tween the result of the formula and the mean of Professor Forbes' s own 

 observations ; the errors in the two cases are however of opposite 

 kinds. 



The manner in which the observations in table III. were made, has 

 been already noticed. The calculated altitudes of the sun at the period 

 of the first observation on 7th January, and at that of the last on 30th 

 April, were respectively 51° and 88 o 17', Supposing the atmosphere 

 equally pervious, the ratio of the intensity at the former angle to that 

 at the latter, is theoretically a little greater than 9-tenths, a ratio ap- 

 proaching much nearer to equality than it would have done if the alti- 

 tudes had been smaller with the same common difference. It is re- 

 markable that on the 14th January the photometer indicated 37, the 

 greatest intensity recorded, and on the following day 32, the only other 

 observation as great as the latter of these having been on the 30th 

 April. It suggested itself to me at one time as probable that the ap- 

 parent effect of the sun's rays might be influenced by the temperature 

 of the instrument employed to measure it, but there can be no doubt 

 that this and every other disturbing cause is excluded in the results 

 obtained by means of the actinometer. The varying quantity of undis- 

 solved vapour lying in the path of the rays, offers perhaps the best ex- 

 planation of the anomalous variations in their intensity, as was suggest- 

 ed by Dr. Richardson in explanation of the apparently great power of 

 the sun's rays at Fort Franklin. Successive strata of thin vapour^at a 

 great height, may be conceived to intercept many of the heating rays, 

 without causing a diminution of transparency sensible to the eye. 



I regret extremely that the tables contain no observations on terres- 

 trial radiation at night. The method of using the actinometer in 

 the measurement of solar radiation, as already described, is applicable 

 also to terrestrial radiation. Of course in this case the glass plate be- 

 tween the bulb of the instrument and the sky would need to be remov- 

 ed : it would also be necessary to lengthen the time of observation, and, 



* Transactions of the Royal Society of Edinburgh, vol. *xii. 



