BRITISH ASSOCIATION. 49 



placed horizontally on a stand, and revolving on its own axis by menne of wheels 

 inside this tube a telescope or an opera glass is placed, by which, by means of 

 two opposite screws, the end of the object-glass can be placed in an excentric 

 position in various degrees according to the eflFect desired, while the eye-glass re- 

 mains in the centre of the small end of the tube. Now, if we understand that 

 when the machine makes the tube to revolve upon its axis, the telescope inside 

 revolves in an excentric direction, during the revolution the star seen through it 

 must appear like a circle. This circle exhibits on its periphery the various raya 

 emitted by the star, all following each other in spaces corresponding with their 

 duration, showing also blank spaces between two contiguous rays which must cor- 

 respond with the black lines of the spectrum. The instrument, in fact, is a kind 

 of spectroscope, by which we can analyze the light of any star, study the cause of 

 the scintillation, and compare its intensity in various climates or seasons and at 

 different altitudes. 



The Abbk Moigno exhibited and described M. Soleil's Tenebroscope, for illus- 

 trating the invisibility of light. . It is well known to scientific men, although the 

 general public do not sufficiently appreciate the fact, that light in itself is invisi- 

 ble unless the eye be so placed as to receive the rays as they approach it, or unless 

 some object be placed in its course, from whose surface the light may be reflected 

 to the eye, which will generally thus give notice of the presence of that object- 

 Thus, if the strong beam of sunlight be admitted into a darkened chamber through 

 a email opening and received on some blackened surface placed against the oppo- 

 site wall, the entire chamber will remain in perfect darkness, and all the objects 

 in it invisible, except in as far as small motes floating in the air mark the course 

 of the sunbeam by reflecting portions of its light. Upon projectino- a fluid or 

 small dust across the course of the beam its presence also becomes perceptible. 

 The instrument exhibited consisted of a tube with an opening at one end to be 

 looked into, the other end closed, the inside well blackened, and a wide opening 

 across the tube to admit strong light to pass only across. On looking in all is 

 perfectly dark, but a small trigger raises at pleasure a small ivory ball into the 

 course of the rays, and its presence instantly reveals the existence of the crossing 

 beam by reflecting a portion of its light. 



' On the System of Forecasting the W eather pursued in Holland,' by Dr. Buys 

 Ballot. — The author said: — " I shall not abuse your indulgence, which I earnestly 

 implore. I shall very shortly explain (1.) what are the rules about foretelling 

 weather in Holland, given before a similar system was introduced in England • 

 (2.) how they behaved themselves; and (3.) what is to be done now: and I will 

 very abundantly answer to any question or remark if they be made, for in that 

 case I am justified in trespassing on your time. — (1.) Under our plan, where ob- 

 servations are taken in Holland, tb ere are four principal places : Helder indicated 

 by H, Groningen indicated by G, Flushing indicated by V, and Maestricht indica- 

 ted by M, on the indications of which I base my forecasts, and in the first place on 

 the barometer readings. For every day of the year and for every hour of the day 

 I have very carefully determined the height of the barometer in the place of ob- 

 servation at that height above the sea, where it is suspended. This is a cardinal 

 point not sufficiently observed in England, and not at all in France. The difler- 



VOL. IX. D 



