1868.] On Physical Observations to he undertaken in India. 283 



In such differential systems a constant force, H, sufficient to keep the 

 governor in a proper state of efficiency, is appUed to the axis 77, and the 

 motion of this axis is made to work a valve or a break on the main shaft 

 of the machine. S in this case is merely the friction about the axis of |. 

 If the moments of inertia of the different parts of the system are so 

 arranged that M' = 0, then the disturbance produced by a blow or a jerk 

 on the machine will act instantaneously on the valve, but will not commu- 

 nicate any impulse to the governor. 



II. " Proceedings of the Council of the Royal Society with refer- 

 ence to the undertaking of certain Physical Observations in 

 India/^ Communicated to the Society by direction of the Pre- 

 sident. 



On the 13th of February 1866, J. S. N. Hennessey, Esq., First Assistant 

 on the Trigonometrical Survey of India, addressed a letter to the President, 

 in which, after explaining the nature of the calls upon his time occasioned 

 by his professional duties, he offered to devote any portion of his leisure 

 to such scientific experiments as the President might direct. He stated 

 that he was resident at Mussoorie from May to October, residing at Dehra 

 during the remainder of the year, from which place, however, he would be 

 able to visit Mussoorie occasionally, such as once a week. Mussoorie is a 

 hill-station or town, at an altitude above the sea-level of about 6700 feet, 

 in lat. 30° 28' N., and long. 78° 10' E. With reference to the climate of 

 this place he observes, " In September the skies begin to clear, and 

 from September 15 to about December 15 there prevails at Mussoorie 

 a clearness of atmosphere such as I have never known in my wanderings. 

 I mention this period in particular ; but at all times, when the sky is clear 

 of clouds, the intensity of the heavenly bodies is something exquisite. I 

 have seen Venus distinctly at midday with my unaided eye. It is this 

 wonderful transparency of atmosphere to which I would draw particular 

 attention." 



This letter was read to the Council on the 28th of June, and was ordered 

 to be printed, and the President was requested to communicate it to such 

 men of science as he might see fit, with the view of obtaining suggestions 

 with reference thereto. 



The President accordingly communicated the letter to several scientific 

 men, accompanied by a request that they would favour him with suggestions 

 as to observations which it might seem to them desirable that Mr. 

 Hennessey should be requested to take up. Answers were received during 

 the recess ; and on the 1 1th of October a Committee was appointed to draw 

 up a report upon Mr. Hennessey's letter, giving specific indications as to 

 the observations in meteorology and with the spectroscope which it might 

 be desirable to make. 



Before the Committee presented their report another subject of great 

 scientific interest presented itself in connexion with India. On the 1 7th of 



