396 



NA TURE 



[August 24, 1893 



Office during this period show that the conditions were 

 mostly anti-cyclonic, both over this country and the 

 Continent, with the exception of a depression in the 

 south-west, which caused so ne sharp thunderstorms on 

 the 9th and loth. On the i8th another depression 

 appeared off our north-west coasts, causing a gale in 

 those parts, while strong winds and lightning occurred 

 generally, with heavy rain in the west. These conditions 

 checked the excessive heat ; on the 19th the maximum 

 temperature in London was 15°, and at Paris 25', lower 

 than on the previous day. 



A SENSITIVE SPHEROMETER. 



"T^HE ordinary spherometer has three arms carrying 

 ■'■ three fixed points, with a point moved by a screw 

 in the centre. This form is an improvement on the 

 original spherometer invented by Andrew Ross, and for 

 which the Society of Arts gave him a silver medal 

 in 1841. 



A description of Ross's instrument is given by Holtz- 

 apffel, vol. iii. p. 1271 of his work on "Turning and 

 Mechanical IManipulalion," extracted from vol. liii. of 

 the Transactions of the Society of Arts. This instrument 

 could measure to lijoi) '^^ ^^ inch, and by estimation 

 half this amount. An ordinary spherometer, with a screw 

 of yjjj of an inch pitch and head divided to hundredths, 

 will measure to loJoo of an inch. 



I pointed out in vol. 1. page 145, of the Memoirs of the 

 Royal Astronomical Society that the sensitiveness of the 

 ordinary spherometer was much increased by placing 

 the screw not in the centre, but in one of the arms in 

 place of one of the fixed points ; this at once increased 

 the sensitiveness of the screw in proportion to the dis- 

 tance of the screw from the nearest fixed point, and this 

 fixed point from a line joining the other two fixed points. 



The improvement I wish to bring before those in- 

 terested in spherometers by this note, is the extension of 

 this principle, for by carrying the middle point much 

 nearer the line joining the other two, a proportionate in- 

 crease of sensitiveness is obtained. 



In the case of an instrument I have made on this plan 

 I have increased the sensitiveness thirty times, the dis- 

 tance from the middle point to the screw being three 

 inches, and the distance of the point from the line of the 

 other two being ^Vi "f '^'^ inch ; with a screw of one 

 hundred threads to the inch and a head divided to hun- 

 dredths, the ordinary form of instrument will read to 

 but on the plan I give, the same screw will 



tijoijij 



measure -jfyoWcr °f ■*" inch. 



There is an additional advantage in this form, that 

 the curvature of a part nearly in a line is measured, so 

 that cross measures can be taken. 



The form of the instrument is not symmetrical, and it 

 requires to be balanced, so that when the screw is raised 

 it will be possible to estimate the frictional contact of 

 the outside points when the middle one is taking the 

 weight. This balancing is easily done by adding a handle 

 to the part opposite the arm carrying the screw ; in 

 practice it is found that this handle is of the greatest 

 value in keeping theheat of thehand from the instrument, 

 as even with the ordinary instrument, holding it for a 

 short time in the hand alters the readings materially. 



It is of great advantage to have on the arms carrying 

 the two outer pins two pieces of wood or ivory project- 

 ing not quite as much as the measuring points, so that 

 by tilting the instrument up these two pieces come first 

 into contact with the surface to be measured, then by 

 gradually raising the handle the points are brought gently 

 into contact. The figure is a plan of this spherometer, 

 and shows the position of the three fixed points P P P 

 with reference to the measuring screw s, and the position 

 of the balancing handle H with reference to the un- 



NO. 1243, VOL. 48] 



symmetrical arm carrying the measuring screw ; x x are 

 the projecting pieces already mentioned. 



The movement of the screw being so large for a slight 

 curvature, this instrument is more particularly useful for 

 measuring the slight curvatures of so-called plane mirrors, 



for which, indeed, it was designed. To make it available 

 for measuring differences between parts of a curved sur- 

 face of considerable curvature the middle pin should be 

 a screw capable of movement to, and clamping, in a 

 position, that will allow the measuring screw to work. 



A. A. Common. 



JEAN DANIEL COLLADON. 



pvANIEL COLLADON, the celebrated physicist and 

 •L-' engineer, died on June 30, at Cologny, near Geneva. 



CoUadon was born at Geneva, December 15, 1802. 

 He belonged to a Protestant family from Berry, which 

 removed from France, in the middle of the sixteenth 

 century, on account of religious persecutions, and 

 found refuge in Calvin's town. Many a distinguished 

 magistrate came from this family, amongst others the 

 learned jurls-consult, Germain Colladon. 



While still quite young Colladon proved to be wonder- 

 fully intelligent, and had a remarkably observant mind. 



He went through the College and then the Academy of j 

 Geneva, which at that time had, among its professors, A.] 

 P. de Candolle, M. Aug. Pictet, Th.de Saussure and Pre- 

 vost. His liking for science could not but develop itself 

 in contact with these eminent men, whose esteem he 

 soon gained. 



At the age often years he made friends with Charles 

 Sturm, who became a noted mathematician, and was 

 on later occasions his fellow-worker. His inventive 

 nature and talent for experimental inquiry turned itsell 

 above all to physics and its mechanical applications. 



He was just twenty-two when he received from the 

 Society of Science of Lille a first prize for the inventior 

 of a new photometer. At twenty-three he went to finisi 

 his studies at Paris. He lived there for about ten years 

 leading a simple life, almost entirely devoted to work. 



He was received in a most flattering manner for such, 

 young man by the pleiades of celebrated men, which th( 



