154 PHILOSOPHICAL TRANSACTIONS. [ANNO 1/8 1 . 



appeared to me, as well as to several other gentlemen, to be the darkest venous 

 blood we had ever seen. 



From this experiment, compared with those which have been recited before, 

 we may perceive the reason why animals preserve an equal temperature, notwith- 

 standing the great variations in the heat of the atmosphere, arising from the 

 vicissitudes of the weather, and the difference of season and climate : for, as 

 soon as, by exposure to external cold, an unusual dissipation of the vital heat is 

 produce!, the blood, in the course of the circulation, begins to be more deeply 

 impregnated with the phlogistic principle. It will therefore furnish a more 

 copious supply of this principle to the air in the lungs, and will imbibe a greater 

 quantity of fire in return. In summer, on the contrary, the reverse of this will 

 take place, less phlogiston will be attracted in the minute vessels, and less fire 

 will be absorbed from the air. And hence the power of generating heat is in all 

 cases proportioned to the demand. It is increased by the winter colds, dimi- 

 nished by the summer heats : it is totally suspended or converted into a contrary 

 power, as the exigencies of the animal may require. From the changes which 

 are produced in the colour of the venous blood by heat and cold, we may also 

 perceive the reason why the temperature of the body is frequently increased by 

 plunging suddenly into cold water, and why the warm bath has such powerful 

 effects in cooling the system, and in removing a general or partial tendency to 

 inflammation. 



XXXII. Account of a Comet. By Mr. Herschel, F. R. S. p. 492. 

 On Tuesday the 13th of March, 1781, between 10 and 11 in the evening, 

 while examining the small stars in the neighbourhood of H Geminorum, I per- 

 ceived one that appeared visibly larger than the rest : being struck with its un- 

 common magnitude, I compared it to H Geminorum and the small star in the 

 quartile between Auriga and Gemini, and finding it so much larger than either 

 of them, suspected it to be a comet. I was then engaged in a series of obser- 

 vations on the parallax of the fixed stars, which I hope soon to have the honour 

 of laying before the r. s. ; and those observations requiring very high powers, I 

 had ready at hand the several magnifiers of 227, 400, 932, 1530, 2010, &c. all 

 which I have successfully used on that occasion. The power I had on when 1 

 first saw the comQt was 227. From experience I knew that the diameters of the 

 fixed stars are not proportionally magnified with higher powers, as the planets 

 are ; I therefore now put on the.powers of^46o and y32, and found the diameter 

 of the comet increased in proportion to the power, as it ought to be, on a sup- 

 position of its not being a fixed star, while the diameters of the stars to which I 

 compared it, were not increased in the same ratio. Also, the comet being mag- 



