Proceedings of the Polytechxic Association. 9S1 



average weights of a spoonful of different kinds of tea, it is to be 

 inferred that the quantity of black tea used as compared with that of 

 green is as three to two. 



A XEW,.PnOTOJLETEK. 



The instniraent described by Mr. Eoger Wright, befoi'e the Roval 

 Society, consists of a smooth metallic"r<Ml standing on a heary base. 

 The rod is graduated from a zero point at the base, and the top is 

 painted white with a black spot in the center. Over the whole 

 length of the rod is a closely fittiiig sliding tube blackened on the 

 inside. To a&eertain the strength of diffused sunlight, the observer 

 looks steadily at the black spot, at the same time drawing tlie tube 

 iipward. When the spot has entirely vanished in the gloom, the 

 observer notes the distance at the bottom of the tube from the zero 

 point as shown by the scale, and this is tlie measure of thedntensity 

 of the light. This photometer, like others made on the game prin- 

 ciple, depends on the accuracy of the observer's eye, and the same 

 instrument should therefore always be used by the same person. 



The Transit of Mercuey and its Abpaeent Elongation. 



At a recent meeting of the Hoyal Astronomical Society, in Lon- 

 don, Prof, Airy said he believed the last transit of Mercury, had been 

 better observed at Greenwich than anywhere else, for there six-telescopes 

 were brought to bear upon the plienomenon. Mr. Stone was sta- 

 tioned at the great equatorial. Just before the last contact he saw 

 the phenomenon known as " the black drop," an apparent elongation 

 of the disc of the planet in the direction of the ;linib of the sun. 

 This phenomenon is caused by irradiation : and he thought it might 

 be explained by drawing a diagram, rapresenting, by tw© curved lines, 

 the real and the apparent limb of the ■emi,, a&^enlarged by iirradiation. 

 Touching the inner curve, draw a small leircl^ rrepresentiug the true 

 disc of Mercury, and within this a smaller circle rrepresenting the 

 planet. as it appears when diminished in size by -irradiation. Now 

 when the real edge of Mercurv touches the real linib-of the sun, as 



0-1/ ' 



thus represented, there is not light enough left at timt place to pro- 

 duce irradiation, because of the Ciisp that is formed;; 'Consequently, 

 tliere is an apparent elongation of tlie planet, which may be represen- 

 ted hj drawing a line from the outer circle to t\\& outer curve, which 

 shows the direction of the black drop. In past years, people were 

 very much perplexed by this phenomenon, and his own early idea on 



