The Rev. H. TMoseley on the Descent of Glaciers. [63 



going on, from the time the lead had been laid down. An attempt 

 made to stop it by driving nails through it into the rafters had failed. 

 The force by which the lead had been made to descend, whatever it 

 was, had been found sufficient to draw the nails*. As the pitch of 

 the roof was only 16^°, it was sufficiently evident that the weight of 

 the lead alone could not have caused it to descend. Sheet lead, 

 whose surface is in the state of that used in roofing, will stand firmly 

 upon a surface of planed deal when inclined at an angle of iiO°+, if 

 no other force than its weight tends to cause it to descend. The 

 considerations which I have stated in the preceding articles led rae 

 *to the conclusion that the daily variations in the temperature of the 

 lead, exposed as it was to the action of the sun by its southern 

 aspect, could not but cause it to descend considerably, and the only 

 question which remained on my mind was, whether this descent 

 could be so great as was observed. To determine this I took the 

 following data : — 



Mean daily variation of temperature at Bristol in the month of 

 August, assumed to be the same as at Leith (Kaemtz, Meteorology, 

 by Walker, p. 18), S°-21 Cent. 



Linear expansion of lead through 100° Cent. -0028436. 



Length of sheets of lead forming the roof from the ridge to the 

 gutter 232 inches. 



Inclination of roof 16° 32'. 



Limiting angle of resistance between sheet lead and deal 30°. 



Whence the mean daily descent of the lead, in inches, in the 

 month of August, is determined by equation (2) to be 



/=232X ^!:^x -0028436 X^J^Iil^' 

 100 tan 30° 



/= -027848 inches. 

 The average daily descent gives for the whole month of August a 

 descent of -863288. If the average daily variation of tempex-ature 

 of the month of August had continued throughout the year, the 

 lead would have descended 10-19148 inches every year. And in the 

 two years from 1851 to 1853 it would have descended 20-38296 

 inches. But the daily variations of atmospheric temperature are less 

 in the other months of the year than in the month of August. For 

 this reason, therefore, the calculation is in excess. For the following 

 reasons it is in defect: — 1st, the daily variations in the temperature 

 of the lead cannot but have been greater than those of the surround- 

 ing atmosphere. It must have been heated above the surrounding 



* Tbe evil was remedied by placing a beam across the rafters near the ridge, 

 SHd doubling the sheets round it, and tixing their ends with spilve nails. 



t Thb may easily be verified. I give it as the result of a rough experiment of 

 my own. I am not acquainted with any experiments on the frittion of lead made 

 with sufficient care to be received as authority in this matter. The friction of 

 copper on oak has, however, been determined by General Morin to be 0-02, and its 

 liinitiug angle of resistance 31° 48'; so that if the roof of liristol Cathedral had 

 been inclined at 'AV" instead of 10^, and had been covered with sheets of copper 

 resting on oak boards, instead of sheets of lead resting on deal, the sheeting 

 would not liave slij)ped by its weight only. 



