J 02 On the Instruments necessary [Aug. 



Fjfth Proposition. — A thick Glass, though as much or 

 more permeable to Light than a thin Glass of worse Quality, 

 allows a much smaller Quantity of Radiant Heat to pass. The 

 Difference is so much the less as the Temperature of' the radiating 

 Source is more elevated. 



This proposition is proved by experiments made with glasses 

 of different thickness employed as a screen ; and its truth is 

 sufficiently established, if these experiments be accurate. This 

 curious fact, that radiating heat becomes more and more capable 

 of penetrating glass, as the temperature increases, till at a 

 certain temperature the rays become luminous, leads to the notion 

 that heat is nothing else than a modification of light, or that the 

 two substances are capable of passing into each other. 



Sixth Proposition. — The Quantity of Heat which a hot 

 Body yields in a given Time by Radiation to a cold Body 

 situated at a Distance, increases, cceteris paribus, in a greater 

 Ratio than the excess of Temperature of the first Body above the 

 second. 



This proposition being at variance with the opinion of Mr. 

 Leslie, and of several other philosophers, M. Delaroche thought 

 it necessary to establish it by a great variety of experiments. 

 These experiments leave no doubt of the fact ; though they are 

 not sufficient to enable us to deduce the rate at which the 

 increase takes place. 



I must acknowledge that this proposition appears to me some- 

 what puzzling. One is at first sight disposed to account for it by 

 the inaccuracy of the thermometer as a measure of heat ; but 

 M. Delaroche, aware of such an objection, has remarked, that 

 if we adopt Mr. Dalton's opinion with respect to the thermo- 

 meter, so far from removing the apparent anomaly, it would only 

 serve to make it greater. 



Article VI. 



On the Instruments necessary for a Travelling Astronomer. By 



Col. Beaufoy. 



(To Dr. Thomson.) 



MY DEAR SIR, Hackney Wick, June 17, 1S1*J. 



Thk following instruments, I think, a travelling astronomer, 

 or naval man, should take with him. First : a good Hadley's 

 sextant, of six inches radius, and which I would have divided 

 into half minutes^ and not nearer. 



