316 Account of some Meteorological Instruments. 



tion of the index, Dr Traill obviates this inconvenience by contracting the' 

 open end of the tube, and bending it slightly downwards, after the introduc- 

 tion of the index. 



Both indices are adjusted, for a fresh observation, by bringing them into 

 contact with the mercury, by means of a small magnet. 



III. Register Thermometer. It has been objected to Rutherford's thermome- 

 ters, that the maxima and minima are obtained by means of fluids of very un- 

 equal expansibilities ; and that, therefore, they require a nice adjustment, or a 

 calculation to make their scales correspond. The principal objection to Six's 

 thermometer is, that the index-springs, however constructed, are very liable 

 to alterations in their resistance to the mercurial column, and frequently im- 

 pede, in a serious degree, the free motion of the indices ; so as to unfit the 

 instrument for delicate observations. 



To obviate these inconveniences, Dr Traill contrived a single register ther- 

 mometer (described in the Library of Useful Knowledge), in which the two 

 indices moved in a horizontal tube, and therefore required no springs to re- 

 tain them in their positions ; but that instrument being found liable to de- 

 rangement from slight movements, he has constructed a modification of the 

 instrument, which is represented in Fig. 4. 



In fact, it consists of his thermometer with the tube bent in the middle, so 

 that the legs make with each other an angle of 120. This form renders the 

 mercurial column less liable to separate ; and the inclination of the arms of 

 the tube is just sufficient to prevent the indices slipping down when the mer- 

 cury retires from them. 



In this register thermometer, as in Six's, the thermometric indications are 

 obtained from the expansions and contractions of alcohol ; the indices are 

 moved by a short mercurial column, which is itself acted on by the changes in 

 the bulk of the spirit ; there is a portion of spirit also in the limb a of the in- 

 strument, as in Six's thermometer ; the slight inclination of its arms allows 

 the indices to rest whenever they have been propelled by the mercury, 

 and renders any kind of spring unnecessary. The indices are iron wires en- 

 closed in slender tubes of coloured glass ; their extremities next the mercurial 

 column give the maxima and minima ; and they are adjusted for each observa- 

 tion by means of a small magnet. 



Report upon a Letter addressed by M. le Baron de Humboldt to 

 His Royal Highness the President of the Royal Society., and 

 communicated by His Royal Highness to the Council. 



To His ROYAL HIGHNESS THE PRESIDENT AND COUNCIL OF THE ROYAL 



SOCIETY.* 



PREVIOUSLY to offering any opinion on the important communi- 

 cation on which we have been called upon to report, we feel that 

 This report is taken from the Athenaeum of March 1837. 



