On a new form of the Mercurial Air-pump. 129 



1842, February 22. — Both on this day and on the 13th, the 

 neutral point was above the horizon, though not visible, being 

 eclipsed or masked by the cause which produces the secondary 

 neutral point. Over a space of 3J° above the sea, the positive 

 bands almost wholly disappear before the negative bands are 

 perceptible, and the neutral point is 5° high when the secon- 

 dary neutral point is distinct in the sea horizon. 



Although I have observed the secondary neutral point more 

 than twenty-two times, it has generally appeared under slightly 

 different forms, varying with the intensity of the new polarizing 

 cause which produces it, and with the point of the horizon where 

 the neutral point rises. 



It is unnecessary to describe these different forms ; I shall 

 mention only an observation made on the 21st of April, 1849, 

 under very favourable circumstances. At 6 h 22 m , when the 

 primary neutral point was about 15° high, the secondary neutral 

 point was 2° 50' high, the negative bands covering a space of 8° 

 or 9° between them, the positive bands being above the sea-line. 

 A fog prevailed to some extent, and above the sea-line there was 

 the dark purplish belt previously mentioned, over which the 

 positive bands were stronger than on the part of the sky above it. 



[To be continued.] 



XVI. On a new form of the Mercurial Air-pump. 

 By Professor Poggendorff*. 



THE mercurial air-pump, which has recently been brought 

 into use again, is essentially a very old instrument ; for 

 only a few years after Otto von Guericke had invented the pis- 

 ton air-pump, the members of the Accademia del Cimento, appa- 

 rently unacquainted with his invention, employed for all their 

 experiments on the behaviour of bodies in the vacuum a straight 

 barometer- tube, of greater length than was needed for measuring 

 the pressure of the atmosphere, and expanded at the top into a 

 sort of receiver, which they filled with mercury and then inverted 

 in a vessel of that metal. 



The numerous defects of this rough contrivance, which we may 

 nevertheless regard as a mercurial pump, and especially the won- 

 derful progress which has been made in the construction of pis- 

 ton air-pumps since the times of Boyle and Papin, during the 

 whole of the eighteenth and nineteenth centuries, have caused 

 the idea of the Florentine Academicians to fall completely into 

 oblivion. 



* Translated by Prof. Foster from the Monatsber. d. Berl. Akad. 1865, 

 p. 158. 



Phil. Mag. S. 4. Vol. 30. No. 201 . Aug. 1865. K 



