Mr. J. P. Harrison on Radiation and Vapour. 283 



clearly in the other. The curves in the woodcut are both drawn 

 on the same scale, ^ an inch representing 100 meteors per five 

 minutes. In looking at the curves, we see that they are extremely 

 like, except that every point on the Cape Town one is about a 

 quarter of an hour earlier than the corresponding point on the 

 British one. Now this appears to me to be easily explained thus. 

 If the earth in its orbit cut that of the meteors at right angles, 

 there would be little difference in the times. But since these 

 orbits cut each other at an angle of about 19°*, the case is 

 very much altered ; and if we take p to represent the perpendi- 

 cular distance between two planes passing through Essex and 

 Cape Town, both parallel to the ecliptic, then the distance 

 passed over between the times that Cape Town and Essex touch 

 the middle of the meteoric band =p cot 19°. Now, from a rough 

 approximation, I find that p = about 5000 miles. Hence, since 

 cot 19°= 2-90421, and the velocity of the earth in its orbit is 

 1134 miles per minute, we have 



s _ 5000 x 2-90421 



v~ 1134 



from which the time / is about 13 minutes. So close an ac- 

 cordance between the calculated and observed times is very re- 

 markable. And what has been said with respect to the time of 

 maximum applies equally well to each point of time. 



I should like to take this opportunity of mentioning an error 

 which I made in drawing the curves in Plate III. in the February 

 Number of the Philosophical Magazine. I by accident made 

 every point of the lowest curve (that of Mr. Talmage) five minutes 

 earlier than it should have been. 



I am, Gentlemen, 



Your obedient Servant, 

 St. Leonard's Hall, St. Andrews, George Forbes. 



March 15, 1867. 



XXXVIII. On Radiation and Vapour. 

 By J. Park Harrison, M.A.\ 



HAVING lately shown that the intensity of insolation near 

 the surface of the ground is increased by the presence of 

 a certain quantity of vapour and cloud J, I wish to submit one or 

 two reasons for hesitation in accepting, at any rate to their full 

 extent, the conclusions at which Mr. Croll has arrived on the 

 subject in the higher regions of the air. 



* As deduced separately by the Astronomer Royal and Sir J. Herschel. 



t Communicated by the Author. 



% Proceedings of the Royal Society, No. 90, p. 356. 



