NATURAL PHILOSOPHY. 203 



ing point of a thermometer depended upon two separate causes, the 

 one being a slow contraction of the bulb, continuing for many months, 

 but ultimately ceasing; and the other being a temporary alteration in 

 the dimensions of the bulb, produced by a sudden and considerable 

 elevation of temperature, which disappeared in two or three weeks. 

 The rise in the freezing point of ordinary thermometers, was probably 

 due to a combination of both these causes ; for, if a thermometer had 

 its freezing point set off soon after being blown and filled, there would 

 be, first of all, the comparatively rapid contraction of the bulb, due to 

 the great heat to which it had lately been exposed, and afterwards, the 

 more gradual contraction which continues for several months. The 

 author recommended opticians, instead of "pointing off" their thermom- 

 eters immediately after being filled, to allow them to rest for a month or 

 six weeks, so as to avoid, at least, the first great change which occurs; 

 but, of course, the longer they were kept the better. Mr. Welsh men- 

 tioned another fact which he had observed in thermometers. He took 

 about fifteen thermometers, and after carefully ascertaining their 

 freezing points, kept them rxposed to the temperature of boiling water 

 for about sixty hours, allowing them afterwards to cool very slowly. 

 It was then found that the freezing point had been permanently raised 

 in all of them, by about 0-3 to 0-4 Fahr. The effect of a subsequent 

 sudden elevation of temperature was exactly as before, to lower the 

 freezing point by nearly 0-2 ; the reading which was found after the 

 long continued boiling being again restored in about a fortnight. He 

 was not yet prepared to say whether any effect would be produced by 

 the boiling in the way of bringing the freezing point of a newly-made 

 thermometer to a permanent position, irrespective, of the temporary 

 alteration caused by a sudden elevation of temperature. 



INSTRUMENT FOR MEASURING EVAPORATION. 



A simple instrument to determine the evaporation from the surface 

 of the earth during a given period, has just been invented by Mr. 

 Newman. A shallow basin connected with a graduated tube below it, 

 has a given quantity of water, previously measured, in this tube, 

 forced into it by displacement by air. At the end of the desired time, 

 the water is again let back into the graduated tube, and its loss by 

 evaporation noted. 



ELASTIC SCALES FOR THERMOMETERS. 



A patent has been granted to Messrs. Mackenzie & Blair, of Glas- 

 gow, for the construction of a thermometer scale, printed with the 

 corresponding references on vulcanized caoutchouc, and thus by its 

 elasticity easily adapted to each mercurial tube. The references 

 actually printed on their scale are over one hundred and fifty. 



