^g6 



NATURE 



\Aug. 3, 1876 



it among their twenty-eight publications marked " offi- 

 cial ; " and the more so inasmuch as its teaching directly 

 tends to overturn the rules which guide seamen in storms 

 and hurricanes, as well as the first principles of atmo- 

 spheric physics. 



GENTILLPS TACHEOMETER 



AMONG the instruments exhibited in the South Ken- 

 sington Loan Collection is one likely to prove of 

 great use in survey-making ; it is the invention of M. 

 Gentilli, an Austrian engineer, and its main purpose is to 

 accomplish rapid surveys (hence its name) of difficult 

 country. Not only does it survey the ground, but it gives 

 the height and distance of every point surveyed. The 

 instrument itself differs little from an ordinary surveying 

 telescope. A vertical lever, A, is attached to the axis of 

 the telescope by means of a screw, c (in figure) ; this lever 

 moves the axis through a given angle, which can be exactly 

 adjusted by means of the two stops, B b', opposite the free 

 end of the lever. The points to be surveyed are marked 

 by a surveying staff, on which are shoivn in a manner to be 

 visible at a great distance, very minute divisions of a foot. 

 The telescope is pointed to this distant staff of which it 

 measures : {a) the horizontal angle of position, {b) the ver- 

 tical angle of elevation, (<r) the distance of the instrument 

 from the staff. It is the accuracy with which this last 



within ^^^ part. The instrument seems likely to be of 

 the greatest use both to ordinary surveyors and to those 

 who have to carry on extensive topographical operations. 



datum can be read that is accomplished by the peculiar 

 mechanism of Gentdli's instrument. As an example : 

 Suppose the staff marked with divisions, to have a scale 

 of 12 feet, on which feet, inches, and eighths of an inch 

 are shown. The telescope is directed to the top of the 

 scale, of which it gives the horizontal and the vertical 

 angle. It is next directed downwards by the screw to a 

 fixed stop, and there it reads on the staff, say 10 feet 

 5 inches below the former reading ; that distance on the 

 staff is i,coo eighths of an inch, and tells us that the staff 

 is 2,000 yards off. In short, the greater the angle through 

 which the telescope is moved, the greater the distance 

 and vice versa, Gentilli's telescope reading the distance 

 and giving it exactly as read, without calculation of any 

 kind. The mechanism is so precise that the telescope 

 can be moved through any given angle and restored to 

 its original position with almost perfect accuracy. Prac- 

 tice has shown that the distances so measured by a small 

 instrument of only 40 magnifying power are correct to 



THE RADIOMETER IN FRANCE 



A LTHOUGH Mr. Crookes's apparatus was described 

 ■^^ in a few French papers at the end of last year, the 

 novelty of the phenomenon has prevented physicists from 

 paying due attention to it till within the last three months. 

 But now the subject has been brought before the Institute 

 and a number of experiments have been made or axe 

 being contemplated which are deserving of notice. 



The first apparatus in Paris were procured from London, 

 and also from Germany by Geissler ; but now they are 

 exported from Paris. There are in Paris not less than 

 three makers— M. Gaeffe, M. Alvergniat, and M. Saleron 

 — who are daily selling the apparatus, so that the instru- 

 ment will soon become common in all laboratories in 

 spite of the price, which is about 25 francs. 



M. Fizeau, the well-known physicist, has stepped 

 forward to defend the theory of air-dilatation. The most 

 formidable objection was proposed by M. Ledieu before 

 the Institute. This mathematician insists upon the great 

 fact that in the air at the ordinary pressure the blackened 

 plate is attracted instead of being repelled. He says that 

 there is a decided opposition between these two pheno- 

 mena, and that at a certain pressure the radiometer 

 cannot move at ail. I do not know whether the experi- 

 ment has been actually tried. The best plan for investi- 

 gating the question is to construct radiometers in different 

 gases, carbonic acid and hydrogen, which I intend to do. 

 If the rotation is produced merely by dilatation of the 

 residual gas the motion must be quicker in hydrogen and 

 slower in carbonic acid, owing to the difference of con- 

 ducting power and mobility. But even then it remains 

 to account for the inversion of rotation. 



Sometimes the radiometer rotates in an opposite direc- 

 tion without any apparent cause operating upon it. In 

 investigating the question I demonstrated very simply 

 that this is because it emits heat. To obtain inverse 

 rotation it is sufficient to leave it for some length of time 

 exposed to the rays of the sun, or to the radiation of a 

 furnace, and to plunge it in a vessel full of cold water. 

 The effect is immediate, the inversion takes place almost 

 instantly ; but the real quantity of heat accumulated in 

 plates being very small indeed, the inverse rotation is 

 accelerated for a few seconds, and diminishes at a very 

 rapid rate. In less than half a minute the radiometer stops 

 and direct action of the rays causes it to rotate again in the 

 direct way if the vessel is of glass and transparent. The 

 same experiment can be made in the shade, but it requires 

 more caution, as the inverse action is less powerful, and 

 the light can operate with sufficient force to continue the 

 rotation in the normal direction, in spite of reverse force 

 produced by refrigeration. But even in these cases it is 

 possible to perceive a diminution in the rate of rotation. 

 The radiometer falls to a rate which is smaller than the 

 final one, and suffers a visible augmentation after a tem- 

 porary diminution in the first instance. 



M. Alvergniat exhibited, at a recent sitting of the 

 Socidt^ de Physique, a double apparatus to demonstrate 

 that the position of the blackened face determines the 

 direction of rotation. The following experiments can be 

 made with a radiometer with both plates blackened, and 

 illustrate the same fact with greater simplicity if the half 

 of the transparent sphere has been previously blackened. 



If the blackened hemisphere is perpendicular to the 

 rays, the radiometer will remain motionless ; but in an 

 oblique direction it will rotate to the left or to the right, 

 according to the inclination of the incident rays. The least 

 surplus in the quantity of light or heat received by any 

 influenced surface will rotate the apparatus in the direc- 

 tion of repulsion. 



