370 Double Stars. 



globe and receiver M. Lamont found to be compounded of that 

 of the heated air and of the aqueous vapour with which the 

 serial space of the globe was saturated (by convection) . This 

 experiment is proof sufficient, in the opinion of M. Lamont, 

 that the particles of vapour and of air are not, as in the theory 

 of Dalton, indifferent one to another as grains of dust, but 

 exert upon one another a mutual and permanent reaction. 

 Whereas the aqueous particles are incapable, at low tempera- 

 tures, to exert those pressures by which they might assume 

 among the particles of air positions of equal and independent 

 action, M. Lamont advocates a view that an atomic combina- 

 tion arises between these diverse particles, causing to the satu- 

 rated air a character of humidity. This character he believes 

 to be imparted to the air by actual contact only with the source 

 of vapour, and not by any transfer of the particles of vapour 

 among the particles of air. 



DOUBLE STARS.— OCCULTATIONS.— THE EARTH IN 

 OPPOSITION. 



BY THE EEV. T. W. WEBB, P.E.A.S. 



DOUBLE STAES. 



We will return to the constellation Cygnus, before it passes 

 away too far to the west, for the sake of a very inconspicuous 

 object, but, at the same time, one of the most remarkable in 

 the heavens — a double star, whose name, 61 Cygni, will ever be 

 henceforth associated with a most memorable epoch in sidereal 

 astronomy. The unusual amount of the common proper motion 

 discovered by Piazzi, in this pair, 5"'2 in R. A. and 3'"2 in D. 

 annually, or 1° in 700 years, induced the late eminent observer 

 Bessel to suppose that it might be at a less impracticable dis- 

 tance from the earth than its neighbours, and might indicate 

 that distance by a sensible parallax. He therefore undertook 

 this most delicate and difficult investigation with the great 

 heliometer* at Konigsberg, measuring, at different seasons of 

 the year, the interval between the pair and two smaller stars 



* The instrument so called, or rather miscalled, is an achromatic telescope, the 

 object-glass of which, after its completion, is cut across into two halves, each so 

 mounted that the straight edges are capable of sliding against one another, in 

 obedience to a screw movement, the handle of which is brought within reach of 

 the observer. So long as the two halves maintain the same position which they 

 had before bisection, they produce a single image at the focus like an ordinary 

 object-glass ; but any lateral displacement has the immediate effect of converting 

 this single image into two, whose distance can be varied at the pleasure of the 

 observer ; and in the same way the images of two distant objects in the same 

 field cBn be made to coincide, and thus micrometrical measurements can be 



