TRANSACTIOXS OF THE ilECTTOXP. 59 



Instetjments. 



On the Mensurator, a new Instrument for the Solution of Triangles. 

 By W. Maesham Adams. 



The Mensurator is an instrument by which triangles may be solved ; that is to 

 say, when the necessary data are given, the instrument maybe set according to 

 them, and the values of the other parts read off. 



The author illustrated the use of the instrument by numerous examples. 



On a neiv Hygrometer. By George Dines, 



The peculiarity of this instrument is, that the use of ether is altogether dispensed 

 with, water only of a lower temperature than the dew-point beiug required : this 

 is placed iu a small reservoir., By turning a tap, the water is allowed to pass through 

 a closed vessel covered with thin polished metal or black glass. As soon as the 

 dew appears \ipon the surface of the metal or glass, the ilow of the water must be 

 stopped ; and a thermometer, the bulb of which is enclosed in the vessel, shows the 

 temperature of the dew-point. 



Using this instrument side by side with the dry- and wet-bulb thermometers, 

 the author of the paper lias come to the conclusion that (whatever Tables may be 

 used) the latter can never be depended upon as giving more than an approximation 

 to the dew-point. He also describes the aqueous vapour in the atmosphere as badly 

 mixed ; and that masses of air very differently charged with moisture are rolling 

 over the surface of the earth in the same manner as the clouds above^ the difference 

 being that they are invisible. 



On a Nautical Photometer. By J. Hopkinsoit, D.Sc. 



The photometers hitherto used for viewing distant lights, too faint for comparison 

 with a standard caudle, depend on the absorption of light by a coloured medium 

 till the light is no longer visible. These photometers are defective : — 1st, as they 

 do not give the true factor by which the light is diminished in intensity : 2nd, the 

 absorbing medium is arbitrary, and must be arbitrarily defined ; 3rd, the eflects on 

 red and white lights are not comparable. 



The photometer suggested consists of two suitably mounted Nicol's prisms, 

 which can be turned about a common axis till the light is eclipsed. If x be the 

 angle between the polarizing planes of the prisms, the light must be diminished 



— ^ times to render it invisible, where m is a constant near unity depending on 



the reflection at the faces of the prisms. 



Nouveau Thermom^tre destine a prendre les Temperatures de la Surface des 

 Eaux Marines ou Fluviales. Par le Docteur Janssen. 



J'ai rhonnem- de presenter au meeting un thermometre d'un nouveau modele, 

 destine a prendre la temperature de la surface de la mor ou des fleuves. 



Get instrument dont j'ai deja publie ime description dans les bulletins de la So- 

 ci^te Mi^teorologique de France, le 3 Decembre 1807, a 6t6 employe depuis par im 

 gi'and nombve d'observateurs et a donne des residtats tres-sati.sfaisants, qui permet- 

 tent .de le considerer comme definitivement acquis a la science. 



La disposition nouvelle de cet instrument consiste en ce que le reservoir est 

 place au milieu d'un pinceau de tils de chanvre. Ce pinceau est fixe a la garniture 

 de bois ou de cuivre du thermometre ; il porte a sa partie superieure une virole de 

 plonib. Lorsque I'instrument est jete a I'eau, la virole de plomb I'entrainant, il y 

 p^netre rapidement et verticalement ; les fils de chanvre s'ecartent aussitot et le 

 reserA'oir thermometrique so trouve alors en contact avee le liquide, dont il prend 

 la tempiSrature. En quelques secondes T^quilibre est atteiut et on peut retirer le 



