VOL. XXXII.] PHILOSOPHICAL TRANSACTIONS. 675 



In these works they seldom undermine the ground; but as they dig away the 

 earth below, others are employed to dig and carry off the surface ; otherwise 

 the matter above, being of so light a nature, would fall in and endanger the 

 workmen; for as that stratum of sand-stone, which occurs before they come to 

 the fullers-earth, does not lie, as in coal-pits, immediately over the matter they 

 dig for, like a ceiling, but even in the midst of the superjacent strata of sand, 

 and therefore can be no security if they undermine. 



The perpendicular fissures are frequent, and the earth in the strata, besides 

 its apparent distinction into layers, like all other kinds of matter, by reason of 

 its peculiar unctuousness, or the running of the adjacent sand imperceptibly 

 among it, breaks into pieces of all angles and sizes. 



These pits lie in that ridge of sand-hills near Woburn, which near Oxford 

 is called Shotover, on which lies Newmarket heath by Cambridge, and which 

 extends from east to west every where, at about the distance of 8 or lo miles 

 from the Chiltern hills, which in Cambridgeshire are called Gog-magog; but 

 in Bucks and Oxon, the Chiltern hills, from the chalky matter, of which they 

 chiefly consist; which two ridges you always pass in going from London into 

 the north, north-east, or north-west counties. After which you come into 

 that vast vale, forming the greater part of the midland counties of Cambridge, 

 Bedford, Bucks, Northampton, Oxford, and Gloucester. 



An Invitation for making Meteorological Observations. By Dr. James Jurin 

 R. S. Seer. W 379, P- 422. Abridged from the Latin. 



Dr. Jurin states that the changes in the weather, especially when great or 

 sudden, have much influence on the health of mankind; for which reason phi- 

 losophers, even in the ]7th century, invented various instruments, by which 

 were ascertained the several degrees and changes in the weight, heat, moisture 

 and elasticity of the atmosphere. They endeavoured also to discover the causes 

 of these changes, adding several observations on the weather, the face of the 

 sky, the winds, and quantity of rain. 



Were this done more generally, and the observations compared, we should 

 have a still more perfect history of the air. In general, the sudden changes in 

 the weather are chiefly to be attributed to the winds; hence then we should 

 have some means of evincing the cause of the winds, and in particular to deter- 

 mine the truth or falsehood of Dr. Halley's opinion, in N° 181, who thinks 

 that the ascent of the mercury in the barometer is owing to the winds blowing 

 towards the same place from opposite points, thus collecting and accumulating 

 the air ; as, on the contrary, that its descent is caused by the winds carrying the 



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