the Vapour Attnosphere at Bombay. 247 



also of ascertaining what were the relative amounts of water 

 evaporated during the different portions of the time, — the 

 cooling being the effect of the conversion of certain quantities 

 of water into aqueous vapour, and being in every separate 

 part of that time proportioned to the quantity converted. 



We have also in a separate column of Dr. Buist's returns, 

 the difference between the dry and the wet thermometers; 

 and this difference may be taken to express the relative quan- 

 tities of water evaporated, and the force of evaporation. From 

 about six in the morning, varying with the season, this differ- 

 ence generally increases until, say twelve or one o'clock in the 

 day, when it declines until the following morning. 



As already stated, evaporation is less in the wet than in the 

 dry season ; and there is also less difference between the morn- 

 ing and mid-day force of evaporation in the former than in 

 the latter season. The cold part of the year is the period for 

 the land and sea breezes, and also for the extensive daily fluc- 

 tuations of the barometer; and Dr. Buist says that this season 

 lasts five months, namely, during October, November, De- 

 cember, January and February, forming the winter of the 

 part. But of these October is wavering and uncertain ; we 

 may therefore consider the other four months as the season 

 when the sea and land breezes blow in the most decided man- 

 ner, and in which the greatest daily fluctuations of the baro- 

 meter take place ; it is consequently desirable that we should 

 carefully examine the returns for the whole of these months. 

 To do this, we will take the mean hourly difference between 

 tlie dry and wet thermometers for each month, and by adding 

 them together and dividing by four, obtain the mean hourly 

 force of evaporation for the winter season. This I have done, 

 and the result is given in the following table, to which are 

 added in other columns the mean hourly heights of the baro- 

 meter, thermometer and wet-bulb thermometer, for the same 

 period, that the whole may be seen at one view in juxtaposition. 

 The same facts are shown in a diagram (Plate IV.), in which 

 the curve of the wet-bulb thermometer is exhibited in accord- 

 ance with the figures of the table ; and also as a base line, 

 from which the distance of the dry thermometer is shown, the 

 intermediate space marking the relative force of evaporation 

 in each hour of the day. 



