370' 



NATURE 



\^Augiist I, 1878 



indicated by the movement of the galvanometer needle. 

 A thin strip of hard rubber, placed in the instrument, 

 exhibits extreme sensitiveness, being expanded by heat 

 from the hand, so as to move through several degrees the 

 needle of a very ordinary galvanometer, which is not 

 affected in the slightest degree by a thermopile facing and 

 near a red-hot iron. The hand, in this experiment, is 

 held a few inches from the rubber strip. A strip of mica 

 is sensibly affected by the heat of the hand, and a strip of 

 gelatin, placed in the instrument, is instantly expanded 

 by moisture from a dampened piece of paper held two or 

 three inches away. 



For these experiments the instrument is arranged as in 

 Fig. 2, but for more delicate operations it is connected 

 with a Thomson's reflecting galvanometer, and the current 

 is regulated by a Wheatstone's bridge and a rheostat, so 

 that the resistance on both sides of the galvanometer is 

 equal, and the light-pencil from the reflector falls on 0° of 

 the scale. This arrangement is shown in Fig. i, and the 

 principle is illustrated by the diagram, Fig. 4. Here the 

 galvanometer is at ^, and the instrument which is at / is 

 adjusted, say, for example, to ten ohms resistance. At 

 «, b^ and c the resistance is the same. An increase or 



diminution of the pressure on the carbon button by an 

 infinitesimal expansion or contraction of the substance 

 under test is indicated on the scale of the galvanometer. 



The carbon button may be compared to a valve, for 

 when it is compressed in the slightest degree its electrical 

 conductivity is increased, and when it is allowed to ex- 

 pand it partly loses its conducting power. 



The heat from the hand held six or eight inches from 

 a strip of vulcanite placed in the instrument— when 

 arranged as last described— is sufficient to deflect the 

 galvanometer mirror so as to throw the light-beam com- 

 pletely off [he scale. A cold body placed near the 



vulcanite strip will carry the light-beam in the opposite 

 direction. 



Pressure that is inappreciable and undiscoverable by 

 other means is distinctly indicated by this instrument. 



Mr. Edison proposes to make application of the prin- 

 ciple of this instrument to numberless purposes, among 

 which are delicate thermometers, barometers, and hygro- 

 meters. He expects to indicate the heat of the stars and 

 to weigh the light of the sun. 



KEW GARDENS REPORT 



C IR JOSEPH HOOKER'S Annual Report for 1877 is 

 •--' more than usually bulky, extending to fifty-three 

 pages. The report opens with the number of visitors 

 admitted to the gardens during the year, which amounted 

 to 687,972, a great excess over those of the preceding 

 year. The points of public interest first treated of by 

 the Director are, as might have been supposed, those 

 affecting the agitation to remove the boundary wall along 

 the Richmond Road and to open the gardens at an earlier 

 hour. These points have been so freely discussed of 

 late in the public press that wc need do no more than 

 refer to them. Referring to his visit to North America, 

 Sir Joseph pays a high tribute to the intelligence and 

 courtesy of the people. He says : " I cannot adequately 

 express my sense of the liberality with which travelling 

 facilities and hospitalities of all kinds were accorded to 

 me by public companies and private individuals wher- 

 ever I went in America. The fact of my being con- 

 nected with this establishment [Kew] was a recognised 

 passport, and this even in the remote settlements of the 

 Far West, for I found a reading people everywhere, few 

 of whom had not heard of Kew Gardens. In the Northern 

 States of America the progress of science, and of institu- 

 tions for the instruction of the people in science, occupy 

 a prominent place in the cheap illustrated periodical 

 literature of the masses ; and nowhere on the globe is 

 this literature better or so imiversally read as in the 

 States. It is hence not wonderful that the progress of 

 such establishments as Kew, the British Museum, South 

 Kensington Museum, &c., should be better known 

 amongst all classes of the people there than they are in 

 the United Kingdom generally, and so I found it." 



Under the head of " Interchange of Plants and Seeds," 

 as also under that of "Official Correspondence," a vast 

 deal of information is gathered together on the acclimati- 

 sation, extended cultivation, or further development of 

 useful plants. Thus we find the ipecacuanha {Cephaelis 

 ipecacuanha), a native of Brazil, has been distributed from 

 the Calcutta Botanic Garden to Ceylon, Singapore, Burmah, 

 and the Andamans. Dr. King, however, does not take a 

 very hopeful view of its ultimate success in India, partly 

 on account of its peculiarly slow growth, which tends 

 to prevent its cultivation being taken up with spirit by 

 European planters, and partly on account of its insig- 

 nificant appearance, which does not even excite interest 

 among the planters. Sir Joseph suggests it as "worthy 

 of consideration whether the Indian government would 

 not do well to establish a nursery in some part of our 

 Indian possessions, with the tropical climatic conditions 

 necessary for its growth." In connection with this sub- 

 ject, considering the great value of the ipecacuanha in 

 cases of dysentery, it is satisfactory to note that there 

 seems some prospect of its cultivation being attempted 

 in the native states of Perak, where the soil and climate 

 are considered to be well adapted to its requirements. 

 Regarding the prophylactic virtues of Eticalyptus globu- 

 lus — a subject which has been to some extent ventilated 

 in our own columns — the experiences of the Kew autho- 

 rities do not throw any further light on it. As valuable 

 timber-trees, however, there can be no doubt that many of 

 the species of Eiicalyptus will prove most valuable. Sir 

 Joseph says:— "The merits of the numerous species of 



