VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. 157 



the spheroidal figure of the earth, and going through the parallactic calculations 

 and deductions, I find the difference of meridians between Greenwich and Cam- 

 bridge, by the observations of this eclipse, to be 4 h 44 m 22 s . 



In the transit of Venus, in 1769, the internal contact was observed by Dr. 

 Winthrop at 2 h 47 m 30 s apparent • time, and at the Royal Observatory, at 

 7 h 28 m 57 s apparent time. Allowing the sun's parallax on the day of the transit 

 to be 8".38, I find by calculation from these observations, that the difference of 

 meridians between Greenwich and Cambridge is 4 1 ' 44 m 1 2 s . Taking the mean 

 between the deduction made from the observations of the internal contact of 

 Venus, and of the beginning and ending of the above solar eclipse, the differ- 

 ence of meridians between Greenwich and Cambridge is 4 h 44 m 17 s . This is 

 the difference that I at present take, when I make use of tables fitted to the 

 meridian of Greenwich ; but I should be still glad of more corresponding obser- 

 vations to ascertain this point. 



Some Thermometrical Experiments ; containing, 1 . Experiments relating to the 

 Cold produced by the Evaporation of various Fluids, ivith a Method of Purify- 

 ing jEther. 2. Experiments relating to the Expansion of Mercury. 3. Des- 

 cription of a Thermometrical Barometer. By Tiberius Cavallo, F. R. S. 

 Nominated to prosecute Discoveries in Nat. Hist, pursuant to the Will of the 

 late H. Baker, Esq., F. R. S. p. 500. 



] . On the cold produced by the evaporation of fluids, with a method of purify- 

 ing cether. — It is at present well known, that by the evaporation of various fluids 

 a sensible * degree of cold is produced; and that by the evaporation of aether, 

 which is the most volatile fluid we are acquainted with, water may be congealed, 

 and the thermometer may be brought several degrees below the freezing point. 

 But as various thermometrical experiments, which I lately made, have exhibited 

 some new phenomena, and as I have contrived an easy and pleasing method of 

 freezing a small quantity of water in a short time, and in every climate ; I think 

 it not improper to give an account of these things in the first part of this 

 lecture. 



My first experiments were intended to discover, if possible, a fluid cheaper 

 than aether, by the evaporation of which a degree of cold sufficient for some 

 useful purpose might be generated. B„ut in this my expectation was disappointed, 

 as I found that aether was incomparably superior to any other fluid, as the cold 

 it produced was several degrees greater than that occasioned by any other of the 

 most volatile fluids whatever. Being therefore obliged to use aether, I endea- 

 voured to contrive a method, by which the least possible quantity of it might be 

 wasted in the production of a degree of cold sufficient to freeze water, and in 

 this I met with success. But before we come to the description of this method 



